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Quiz time: Is \@@domain.example a syntactically valid email address? Why (not)?

The local part of an email address is allowed to consist of a single @ character. However, it must be encoded, e.g. as "@"@domain.example. RFC 821 (SMTP) allows using a backslash to escape a character even outside of quoted strings. RFC 822 (internet message format) explicitly states that this syntax is not permitted for the local part of an email address. All successor documents to these RFCs do not permit "backslash escaping" outside of a quoted string in the local part either.

So the answer depends on the context in which the address is used. SMTP servers that still support the obsolete parts of RFC 821 will accept \@@example.com as sender or recipient address. However, in the email's header the address has to be written as "@"@example.com, because \@@example.com is not permitted there.

@cketti no because @ in the local part is only allowed in quoted format. So it would have to be "@"@example.com

Wouldn't surprise me if a bunch of MTAs choked on that though.

@fedops I can confirm that some MTAs do funny things when confronted with "@"@example.com 😀

@cketti 😁

Btw thanks for your work on k9mail! Really appreciate it.

@fedops Don't try this with K-9 Mail. It'll fall over faster than you can say oops 😉

@fedops I'm awarding 2 of 5 points for this answer.

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