Constraints on SMTP Message-Id?

Question

Are there constraints on the length and/or format of SMTP message-id's? I.e.: How long may they be, and are only certain characters allowed? (I plan to use only ASCII, but I fear that there may even be ASCII characters which aren't allowed.)

RFC822 defines this, but are there updated RFCs or common real-World aspects (such as common bugs in mail software) which should be considered?

Answer 1

The updated RFC is RFC2822.

As of characters allowed, it basically ::alpha::|::digit::|[!#$%&'+-/=?^`{}|~.] (It's also possible to quote literals).

It has to contain @ separating "local part" and "domain part". It MUST be globally unique.

Answer 2

If you're looking for an example, you could look at the code for the email.utils.make_msgid Python library function.