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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 <>,
    "" <>,
    "''" <''>,
    "''" <''>,

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 "o'". 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:

  1. Am I correct in my understanding that these addresses are invalid?
  2. 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?
  3. 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?

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Addresses inside <> AFAIK, must not be quoted at all, can be quoted only free-form comment

I have a number of answers

  1. Yes, e-mails 3 and 4 are invalid
  2. In best case - sending MUA (must not produce invalid headers even in case "was received in this form"). First MTA also may (I have re-read for proper verb) check validity of received data and react accordinly
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