One of our users has received an email which has a number of single-quoted addresses in the "To:" header (as well as other addresses), like this:
To: Some One <Sone@example.com>, "firstname.lastname@example.org" <email@example.com>, "'firstname.lastname@example.org'" <'email@example.com'>, "'firstname.lastname@example.org'" <'email@example.com'>,
When our user tried to reply-all, the email was rejected by our mailserver with a response of "501 5.1.3 Bad recipient address syntax".
My understanding of RFC 5322 (which obsoletes RFCs 2822 and 822) is that a single quote (') is valid in the local part of an email address (before the "@"); e.g. as an apostrophe in an address like "firstname.lastname@example.org". However it is not a valid character to use to quote the whole address, and therefore the addresses above are not syntactically correct, and our mailserver was right to reject the reply.
I have a number of questions:
- Am I correct in my understanding that these addresses are invalid?
- Which system has responsibility to prevent a message with invalid addresses in the header being sent? The sender's user agent (Outlook, whatever)? The mailserver the user agent submits the message to? Our mailserver?
- I suspect that the sending SMTP server was MS Exchange; is there a known issue that allows addresses like these?
Depending on the answers to those questions, I expect I'll submit one or more bug reports (to vendors and/or administrators of the sending mailserver), as well as advising our user to remove the single quotes before replying. Any other suggestions for this situation?