MLLP pong is a simple MLLP server for you to test your MLLP integrations. Since you're probably already suffering, this image is as simple as possible for you to make integration tests.
In this server you will have multiple servers that will reply differently to your requests. Depending on what you want to test, use the following ports:
docker build -t mllp-pong .
docker run -p 666:666 -p 1337:1337 mllp-pong
This server will reply with an ack as long as the message sent is valid
This server will reply with an HL7 Internal server error no matter what you do or send
Because the error messages are sent via an ACK message, which requires a valid MSH (message header segment), if you send gibberish you will get an error message that looks like HL7 but is not a valid HL7 message (does not have all the mandatory fields).