diff --git a/MimeKit/MimeVisitor.cs b/MimeKit/MimeVisitor.cs
index 4c29c775fc..ef84a36c09 100644
--- a/MimeKit/MimeVisitor.cs
+++ b/MimeKit/MimeVisitor.cs
@@ -24,11 +24,8 @@
 // THE SOFTWARE.
 //
 
-#if ENABLE_CRYPTO
-using MimeKit.Cryptography;
-#endif
-
 using MimeKit.Tnef;
+using MimeKit.Cryptography;
 
 namespace MimeKit {
 	/// <summary>
@@ -50,7 +47,7 @@ public abstract class MimeVisitor
 		/// Dispatches the entity to one of the more specialized visit methods in this class.
 		/// </remarks>
 		/// <param name="entity">The MIME entity.</param>
-		public virtual void Visit (MimeEntity entity)
+		public virtual void Visit (IMimeEntity entity)
 		{
 			entity?.Accept (this);
 		}
@@ -62,12 +59,11 @@ public virtual void Visit (MimeEntity entity)
 		/// Dispatches the message to one of the more specialized visit methods in this class.
 		/// </remarks>
 		/// <param name="message">The MIME message.</param>
-		public virtual void Visit (MimeMessage message)
+		public virtual void Visit (IMimeMessage message)
 		{
 			message?.Accept (this);
 		}
 
-#if ENABLE_CRYPTO
 		/// <summary>
 		/// Visit the application/pgp-encrypted MIME entity.
 		/// </summary>
@@ -76,7 +72,7 @@ public virtual void Visit (MimeMessage message)
 		/// </remarks>
 		/// <seealso cref="MimeKit.Cryptography.MultipartEncrypted"/>
 		/// <param name="entity">The application/pgp-encrypted MIME entity.</param>
-		protected internal virtual void VisitApplicationPgpEncrypted (ApplicationPgpEncrypted entity)
+		protected internal virtual void VisitApplicationPgpEncrypted (IApplicationPgpEncrypted entity)
 		{
 			VisitMimePart (entity);
 		}
@@ -89,7 +85,7 @@ protected internal virtual void VisitApplicationPgpEncrypted (ApplicationPgpEncr
 		/// </remarks>
 		/// <seealso cref="MimeKit.Cryptography.MultipartSigned"/>
 		/// <param name="entity">The application/pgp-signature MIME entity.</param>
-		protected internal virtual void VisitApplicationPgpSignature (ApplicationPgpSignature entity)
+		protected internal virtual void VisitApplicationPgpSignature (IApplicationPgpSignature entity)
 		{
 			VisitMimePart (entity);
 		}
@@ -101,7 +97,7 @@ protected internal virtual void VisitApplicationPgpSignature (ApplicationPgpSign
 		/// Visits the application/pkcs7-mime MIME entity.
 		/// </remarks>
 		/// <param name="entity">The application/pkcs7-mime MIME entity.</param>
-		protected internal virtual void VisitApplicationPkcs7Mime (ApplicationPkcs7Mime entity)
+		protected internal virtual void VisitApplicationPkcs7Mime (IApplicationPkcs7Mime entity)
 		{
 			VisitMimePart (entity);
 		}
@@ -114,11 +110,10 @@ protected internal virtual void VisitApplicationPkcs7Mime (ApplicationPkcs7Mime
 		/// </remarks>
 		/// <seealso cref="MimeKit.Cryptography.MultipartSigned"/>
 		/// <param name="entity">The application/pkcs7-signature MIME entity.</param>
-		protected internal virtual void VisitApplicationPkcs7Signature (ApplicationPkcs7Signature entity)
+		protected internal virtual void VisitApplicationPkcs7Signature (IApplicationPkcs7Signature entity)
 		{
 			VisitMimePart (entity);
 		}
-#endif
 
 		/// <summary>
 		/// Visit the message/disposition-notification MIME entity.
@@ -127,7 +122,7 @@ protected internal virtual void VisitApplicationPkcs7Signature (ApplicationPkcs7
 		/// Visits the message/disposition-notification MIME entity.
 		/// </remarks>
 		/// <param name="entity">The message/disposition-notification MIME entity.</param>
-		protected internal virtual void VisitMessageDispositionNotification (MessageDispositionNotification entity)
+		protected internal virtual void VisitMessageDispositionNotification (IMessageDispositionNotification entity)
 		{
 			VisitMimePart (entity);
 		}
@@ -139,7 +134,7 @@ protected internal virtual void VisitMessageDispositionNotification (MessageDisp
 		/// Visits the message/delivery-status MIME entity.
 		/// </remarks>
 		/// <param name="entity">The message/delivery-status MIME entity.</param>
-		protected internal virtual void VisitMessageDeliveryStatus (MessageDeliveryStatus entity)
+		protected internal virtual void VisitMessageDeliveryStatus (IMessageDeliveryStatus entity)
 		{
 			VisitMimePart (entity);
 		}
@@ -151,7 +146,7 @@ protected internal virtual void VisitMessageDeliveryStatus (MessageDeliveryStatu
 		/// Visits the message/feedback-report MIME entity.
 		/// </remarks>
 		/// <param name="entity">The message/feedback-report MIME entity.</param>
-		protected internal virtual void VisitMessageFeedbackReport (MessageFeedbackReport entity)
+		protected internal virtual void VisitMessageFeedbackReport (IMessageFeedbackReport entity)
 		{
 			VisitMimePart (entity);
 		}
@@ -163,7 +158,7 @@ protected internal virtual void VisitMessageFeedbackReport (MessageFeedbackRepor
 		/// Visits the message contained within a message/rfc822 or message/news MIME entity.
 		/// </remarks>
 		/// <param name="entity">The message/rfc822 or message/news MIME entity.</param>
-		protected virtual void VisitMessage (MessagePart entity)
+		protected virtual void VisitMessage (IMessagePart entity)
 		{
 			entity.Message?.Accept (this);
 		}
@@ -178,7 +173,7 @@ protected virtual void VisitMessage (MessagePart entity)
 		/// <code language="c#" source="Examples\MimeVisitorExamples.cs" region="HtmlPreviewVisitor" />
 		/// </example>
 		/// <param name="entity">The message/rfc822 or message/news MIME entity.</param>
-		protected internal virtual void VisitMessagePart (MessagePart entity)
+		protected internal virtual void VisitMessagePart (IMessagePart entity)
 		{
 			VisitMimeEntity (entity);
 			VisitMessage (entity);
@@ -191,7 +186,7 @@ protected internal virtual void VisitMessagePart (MessagePart entity)
 		/// Visits the message/partial MIME entity.
 		/// </remarks>
 		/// <param name="entity">The message/partial MIME entity.</param>
-		protected internal virtual void VisitMessagePartial (MessagePartial entity)
+		protected internal virtual void VisitMessagePartial (IMessagePartial entity)
 		{
 			VisitMimePart (entity);
 		}
@@ -203,7 +198,7 @@ protected internal virtual void VisitMessagePartial (MessagePartial entity)
 		/// Visits the abstract MIME entity.
 		/// </remarks>
 		/// <param name="entity">The MIME entity.</param>
-		protected internal virtual void VisitMimeEntity (MimeEntity entity)
+		protected internal virtual void VisitMimeEntity (IMimeEntity entity)
 		{
 		}
 
@@ -214,7 +209,7 @@ protected internal virtual void VisitMimeEntity (MimeEntity entity)
 		/// Visits the body of the message.
 		/// </remarks>
 		/// <param name="message">The message.</param>
-		protected virtual void VisitBody (MimeMessage message)
+		protected virtual void VisitBody (IMimeMessage message)
 		{
 			message.Body?.Accept (this);
 		}
@@ -226,7 +221,7 @@ protected virtual void VisitBody (MimeMessage message)
 		/// Visits the MIME message.
 		/// </remarks>
 		/// <param name="message">The MIME message.</param>
-		protected internal virtual void VisitMimeMessage (MimeMessage message)
+		protected internal virtual void VisitMimeMessage (IMimeMessage message)
 		{
 			VisitBody (message);
 		}
@@ -241,7 +236,7 @@ protected internal virtual void VisitMimeMessage (MimeMessage message)
 		/// <code language="c#" source="Examples\MimeVisitorExamples.cs" region="HtmlPreviewVisitor" />
 		/// </example>
 		/// <param name="entity">The MIME part entity.</param>
-		protected internal virtual void VisitMimePart (MimePart entity)
+		protected internal virtual void VisitMimePart (IMimePart entity)
 		{
 			VisitMimeEntity (entity);
 		}
@@ -253,7 +248,7 @@ protected internal virtual void VisitMimePart (MimePart entity)
 		/// Visits the children of a <see cref="Multipart"/>.
 		/// </remarks>
 		/// <param name="multipart">Multipart.</param>
-		protected virtual void VisitChildren (Multipart multipart)
+		protected virtual void VisitChildren (IMultipart multipart)
 		{
 			for (int i = 0; i < multipart.Count; i++)
 				multipart[i].Accept (this);
@@ -266,7 +261,7 @@ protected virtual void VisitChildren (Multipart multipart)
 		/// Visits the abstract multipart MIME entity.
 		/// </remarks>
 		/// <param name="multipart">The multipart MIME entity.</param>
-		protected internal virtual void VisitMultipart (Multipart multipart)
+		protected internal virtual void VisitMultipart (IMultipart multipart)
 		{
 			VisitMimeEntity (multipart);
 			VisitChildren (multipart);
@@ -282,12 +277,11 @@ protected internal virtual void VisitMultipart (Multipart multipart)
 		/// <code language="c#" source="Examples\MimeVisitorExamples.cs" region="HtmlPreviewVisitor" />
 		/// </example>
 		/// <param name="alternative">The multipart/alternative MIME entity.</param>
-		protected internal virtual void VisitMultipartAlternative (MultipartAlternative alternative)
+		protected internal virtual void VisitMultipartAlternative (IMultipartAlternative alternative)
 		{
 			VisitMultipart (alternative);
 		}
 
-#if ENABLE_CRYPTO
 		/// <summary>
 		/// Visit the multipart/encrypted MIME entity.
 		/// </summary>
@@ -295,11 +289,10 @@ protected internal virtual void VisitMultipartAlternative (MultipartAlternative
 		/// Visits the multipart/encrypted MIME entity.
 		/// </remarks>
 		/// <param name="encrypted">The multipart/encrypted MIME entity.</param>
-		protected internal virtual void VisitMultipartEncrypted (MultipartEncrypted encrypted)
+		protected internal virtual void VisitMultipartEncrypted (IMultipartEncrypted encrypted)
 		{
 			VisitMultipart (encrypted);
 		}
-#endif
 
 		/// <summary>
 		/// Visit the multipart/related MIME entity.
@@ -311,7 +304,7 @@ protected internal virtual void VisitMultipartEncrypted (MultipartEncrypted encr
 		/// <code language="c#" source="Examples\MimeVisitorExamples.cs" region="HtmlPreviewVisitor" />
 		/// </example>
 		/// <param name="related">The multipart/related MIME entity.</param>
-		protected internal virtual void VisitMultipartRelated (MultipartRelated related)
+		protected internal virtual void VisitMultipartRelated (IMultipartRelated related)
 		{
 			VisitMultipart (related);
 		}
@@ -326,12 +319,11 @@ protected internal virtual void VisitMultipartRelated (MultipartRelated related)
 		/// <code language="c#" source="Examples\MimeVisitorExamples.cs" region="HtmlPreviewVisitor" />
 		/// </example>
 		/// <param name="report">The multipart/report MIME entity.</param>
-		protected internal virtual void VisitMultipartReport (MultipartReport report)
+		protected internal virtual void VisitMultipartReport (IMultipartReport report)
 		{
 			VisitMultipart (report);
 		}
 
-#if ENABLE_CRYPTO
 		/// <summary>
 		/// Visit the multipart/signed MIME entity.
 		/// </summary>
@@ -339,11 +331,10 @@ protected internal virtual void VisitMultipartReport (MultipartReport report)
 		/// Visits the multipart/signed MIME entity.
 		/// </remarks>
 		/// <param name="signed">The multipart/signed MIME entity.</param>
-		protected internal virtual void VisitMultipartSigned (MultipartSigned signed)
+		protected internal virtual void VisitMultipartSigned (IMultipartSigned signed)
 		{
 			VisitMultipart (signed);
 		}
-#endif
 
 		/// <summary>
 		/// Visit the text-based MIME part entity.
@@ -355,7 +346,7 @@ protected internal virtual void VisitMultipartSigned (MultipartSigned signed)
 		/// <code language="c#" source="Examples\MimeVisitorExamples.cs" region="HtmlPreviewVisitor" />
 		/// </example>
 		/// <param name="entity">The text-based MIME part entity.</param>
-		protected internal virtual void VisitTextPart (TextPart entity)
+		protected internal virtual void VisitTextPart (ITextPart entity)
 		{
 			VisitMimePart (entity);
 		}
@@ -370,7 +361,7 @@ protected internal virtual void VisitTextPart (TextPart entity)
 		/// <code language="c#" source="Examples\MimeVisitorExamples.cs" region="HtmlPreviewVisitor" />
 		/// </example>
 		/// <param name="entity">The text/rfc822-headers MIME entity.</param>
-		protected internal virtual void VisitTextRfc822Headers (TextRfc822Headers entity)
+		protected internal virtual void VisitTextRfc822Headers (ITextRfc822Headers entity)
 		{
 			VisitMessagePart (entity);
 		}
@@ -385,7 +376,7 @@ protected internal virtual void VisitTextRfc822Headers (TextRfc822Headers entity
 		/// <code language="c#" source="Examples\MimeVisitorExamples.cs" region="HtmlPreviewVisitor" />
 		/// </example>
 		/// <param name="entity">The Microsoft TNEF MIME part entity.</param>
-		protected internal virtual void VisitTnefPart (TnefPart entity)
+		protected internal virtual void VisitTnefPart (ITnefPart entity)
 		{
 			VisitMimePart (entity);
 		}
diff --git a/MimeKit/ParserOptions.cs b/MimeKit/ParserOptions.cs
index 117083144d..212099f355 100644
--- a/MimeKit/ParserOptions.cs
+++ b/MimeKit/ParserOptions.cs
@@ -29,12 +29,9 @@
 using System.Reflection;
 using System.Collections.Generic;
 
-#if ENABLE_CRYPTO
-using MimeKit.Cryptography;
-#endif
-
 using MimeKit.Tnef;
 using MimeKit.Utils;
+using MimeKit.Cryptography;
 
 namespace MimeKit {
 	/// <summary>
@@ -399,6 +396,7 @@ internal MimeEntity CreateEntity (ContentType contentType, IList<Header> headers
 				if (subtype.Equals ("pkcs7-signature", StringComparison.OrdinalIgnoreCase) ||
 					subtype.Equals ("x-pkcs7-signature", StringComparison.OrdinalIgnoreCase))
 					return new ApplicationPkcs7Signature (args);
+#endif
 
 				// application/pgp-encrypted
 				if (subtype.Equals ("pgp-encrypted", StringComparison.OrdinalIgnoreCase) ||
@@ -409,7 +407,6 @@ internal MimeEntity CreateEntity (ContentType contentType, IList<Header> headers
 				if (subtype.Equals ("pgp-signature", StringComparison.OrdinalIgnoreCase) ||
 					subtype.Equals ("x-pgp-signature", StringComparison.OrdinalIgnoreCase))
 					return new ApplicationPgpSignature (args);
-#endif
 
 				// application/ms-tnef
 				if (subtype.Equals ("ms-tnef", StringComparison.OrdinalIgnoreCase) ||
diff --git a/UnitTests/HtmlPreviewVisitor.cs b/UnitTests/HtmlPreviewVisitor.cs
index 5259df2cf8..0a733e1073 100644
--- a/UnitTests/HtmlPreviewVisitor.cs
+++ b/UnitTests/HtmlPreviewVisitor.cs
@@ -33,7 +33,7 @@ namespace UnitTests {
 	/// </summary>
 	class HtmlPreviewVisitor : MimeVisitor
 	{
-		readonly List<MultipartRelated> stack = new List<MultipartRelated> ();
+		readonly List<IMultipartRelated> stack = new List<IMultipartRelated> ();
 		string body;
 
 		/// <summary>
@@ -59,14 +59,14 @@ public string HtmlBody {
 			get { return body ?? string.Empty; }
 		}
 
-		protected internal override void VisitMultipartAlternative (MultipartAlternative alternative)
+		protected internal override void VisitMultipartAlternative (IMultipartAlternative alternative)
 		{
 			// walk the multipart/alternative children backwards from greatest level of faithfulness to the least faithful
 			for (int i = alternative.Count - 1; i >= 0 && body == null; i--)
 				alternative[i].Accept (this);
 		}
 
-		protected internal override void VisitMultipartRelated (MultipartRelated related)
+		protected internal override void VisitMultipartRelated (IMultipartRelated related)
 		{
 			var root = related.Root;
 
@@ -170,7 +170,7 @@ void HtmlTagCallback (HtmlTagContext ctx, HtmlWriter htmlWriter)
 			}
 		}
 
-		protected internal override void VisitTextPart (TextPart entity)
+		protected internal override void VisitTextPart (ITextPart entity)
 		{
 			TextConverter converter;
 
@@ -200,7 +200,7 @@ protected internal override void VisitTextPart (TextPart entity)
 			base.VisitTextPart (entity);
 		}
 
-		protected internal override void VisitMessagePart (MessagePart entity)
+		protected internal override void VisitMessagePart (IMessagePart entity)
 		{
 			// don't descend into message/rfc822 parts
 		}
diff --git a/UnitTests/MimeVisitorTests.cs b/UnitTests/MimeVisitorTests.cs
index 7f2a5063b7..7cd3627322 100644
--- a/UnitTests/MimeVisitorTests.cs
+++ b/UnitTests/MimeVisitorTests.cs
@@ -102,133 +102,133 @@ class MimeVisitorTester : MimeVisitor
 			public int TextRfc822Headers;
 			public int TnefPart;
 
-			protected internal override void VisitApplicationPgpEncrypted (ApplicationPgpEncrypted entity)
+			protected internal override void VisitApplicationPgpEncrypted (IApplicationPgpEncrypted entity)
 			{
 				ApplicationPgpEncrypted++;
 				base.VisitApplicationPgpEncrypted (entity);
 			}
 
-			protected internal override void VisitApplicationPgpSignature (ApplicationPgpSignature entity)
+			protected internal override void VisitApplicationPgpSignature (IApplicationPgpSignature entity)
 			{
 				ApplicationPgpSignature++;
 				base.VisitApplicationPgpSignature (entity);
 			}
 
-			protected internal override void VisitApplicationPkcs7Mime (ApplicationPkcs7Mime entity)
+			protected internal override void VisitApplicationPkcs7Mime (IApplicationPkcs7Mime entity)
 			{
 				ApplicationPkcs7Mime++;
 				base.VisitApplicationPkcs7Mime (entity);
 			}
 
-			protected internal override void VisitApplicationPkcs7Signature (ApplicationPkcs7Signature entity)
+			protected internal override void VisitApplicationPkcs7Signature (IApplicationPkcs7Signature entity)
 			{
 				ApplicationPkcs7Signature++;
 				base.VisitApplicationPkcs7Signature (entity);
 			}
 
-			protected override void VisitMessage (MessagePart entity)
+			protected override void VisitMessage (IMessagePart entity)
 			{
 				Message++;
 				base.VisitMessage (entity);
 			}
 
-			protected internal override void VisitMessageDeliveryStatus (MessageDeliveryStatus entity)
+			protected internal override void VisitMessageDeliveryStatus (IMessageDeliveryStatus entity)
 			{
 				MessageDeliveryStatus++;
 				base.VisitMessageDeliveryStatus (entity);
 			}
 
-			protected internal override void VisitMessageDispositionNotification (MessageDispositionNotification entity)
+			protected internal override void VisitMessageDispositionNotification (IMessageDispositionNotification entity)
 			{
 				MessageDispositionNotification++;
 				base.VisitMessageDispositionNotification (entity);
 			}
 
-			protected internal override void VisitMessageFeedbackReport (MessageFeedbackReport entity)
+			protected internal override void VisitMessageFeedbackReport (IMessageFeedbackReport entity)
 			{
 				MessageFeedbackReport++;
 				base.VisitMessageFeedbackReport (entity);
 			}
 
-			protected internal override void VisitMessagePart (MessagePart entity)
+			protected internal override void VisitMessagePart (IMessagePart entity)
 			{
 				MessagePart++;
 				base.VisitMessagePart (entity);
 			}
 
-			protected internal override void VisitMessagePartial (MessagePartial entity)
+			protected internal override void VisitMessagePartial (IMessagePartial entity)
 			{
 				MessagePartial++;
 				base.VisitMessagePartial (entity);
 			}
 
-			protected internal override void VisitMimeEntity (MimeEntity entity)
+			protected internal override void VisitMimeEntity (IMimeEntity entity)
 			{
 				MimeEntity++;
 				base.VisitMimeEntity (entity);
 			}
 
-			protected internal override void VisitMimeMessage (MimeMessage message)
+			protected internal override void VisitMimeMessage (IMimeMessage message)
 			{
 				MimeMessage++;
 				base.VisitMimeMessage (message);
 			}
 
-			protected internal override void VisitMimePart (MimePart entity)
+			protected internal override void VisitMimePart (IMimePart entity)
 			{
 				MimePart++;
 				base.VisitMimePart (entity);
 			}
 
-			protected internal override void VisitMultipart (Multipart multipart)
+			protected internal override void VisitMultipart (IMultipart multipart)
 			{
 				Multipart++;
 				base.VisitMultipart (multipart);
 			}
 
-			protected internal override void VisitMultipartAlternative (MultipartAlternative alternative)
+			protected internal override void VisitMultipartAlternative (IMultipartAlternative alternative)
 			{
 				MultipartAlternative++;
 				base.VisitMultipartAlternative (alternative);
 			}
 
-			protected internal override void VisitMultipartEncrypted (MultipartEncrypted encrypted)
+			protected internal override void VisitMultipartEncrypted (IMultipartEncrypted encrypted)
 			{
 				MultipartEncrypted++;
 				base.VisitMultipartEncrypted (encrypted);
 			}
 
-			protected internal override void VisitMultipartRelated (MultipartRelated related)
+			protected internal override void VisitMultipartRelated (IMultipartRelated related)
 			{
 				MultipartRelated++;
 				base.VisitMultipartRelated (related);
 			}
 
-			protected internal override void VisitMultipartReport (MultipartReport report)
+			protected internal override void VisitMultipartReport (IMultipartReport report)
 			{
 				MultipartReport++;
 				base.VisitMultipartReport (report);
 			}
 
-			protected internal override void VisitMultipartSigned (MultipartSigned signed)
+			protected internal override void VisitMultipartSigned (IMultipartSigned signed)
 			{
 				MultipartSigned++;
 				base.VisitMultipartSigned (signed);
 			}
 
-			protected internal override void VisitTextPart (TextPart entity)
+			protected internal override void VisitTextPart (ITextPart entity)
 			{
 				TextPart++;
 				base.VisitTextPart (entity);
 			}
 
-			protected internal override void VisitTextRfc822Headers (TextRfc822Headers entity)
+			protected internal override void VisitTextRfc822Headers (ITextRfc822Headers entity)
 			{
 				TextRfc822Headers++;
 				base.VisitTextRfc822Headers (entity);
 			}
 
-			protected internal override void VisitTnefPart (TnefPart entity)
+			protected internal override void VisitTnefPart (ITnefPart entity)
 			{
 				TnefPart++;
 				base.VisitTnefPart (entity);
diff --git a/UnitTests/TextRfc822HeadersTests.cs b/UnitTests/TextRfc822HeadersTests.cs
index 18f1349ab9..5d3c48bacc 100644
--- a/UnitTests/TextRfc822HeadersTests.cs
+++ b/UnitTests/TextRfc822HeadersTests.cs
@@ -43,9 +43,9 @@ public void TestArgumentExceptions ()
 
 		class TextRfc822HeadersVisitor : MimeVisitor
 		{
-			public TextRfc822Headers Rfc822Headers;
+			public ITextRfc822Headers Rfc822Headers;
 
-			protected internal override void VisitTextRfc822Headers (TextRfc822Headers entity)
+			protected internal override void VisitTextRfc822Headers (ITextRfc822Headers entity)
 			{
 				Rfc822Headers = entity;
 			}