Maximum internet email Message-ID length
I'm looking for the maximum character length allowed for an internet Message-ID field for validation purposes within an application. I've reviewed sources such as RFC-2822 and Wikipedia "Message-ID" as well as this SO question, among other various places. The closest answer I can find is "998 characters" because that is the maximum length that the specification allows for each line in an internet message (from RFC-2822), and the Message-ID field cannot be multiple lines.
Is 998 characters the definitive answer? Is there no such limit?
Answer
If there's one thing I've learned about email, it must be that it's a massively distributed system for fuzzing email software. That is, no matter what the RFCs say, you will find emails violating them, some email software coping and some failing. I think most will limp along with the robustness principle in mind.
With that out of the way, I think the maximum RFC compliant Message-ID length is 995 characters.
The maximum line length per the RFC you cite is 998 characters. That would include the "Message-ID:" field name, but you can do line folding between the field name and the field body. The line containing the actual Message-ID would then contain a space (the folding whitespace), "<", Message-ID, and ">". Semantically, the angle brackets are not part of the Message-ID. Therefore you end up with a maximum of 998 - 3 = 995 characters.