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

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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.

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