Finding Messages by ID
If PKB give you a Message ID and a time they received it the way to find it in Mirth is -
Filter the Messages on a time period about 10 minutes before they say they received the file to account for queueing at their end.
Optionally also use the Advanced option to filter the search to just the Message Type you’re interested in.
Put the Message ID in the Text Search box. Mirth will warn you that this will take a while if there are a lot of messages to search.
You should then get back the single message they are referring to.