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?
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.
If you're looking for an example, you could look at the code for the email.utils.make_msgid Python library function.
来源：https://stackoverflow.com/questions/594330/constraints-on-smtp-message-id