[dathinab]

> Note: I have struggled to verify this one, and it’s possible I’m actually misreading the RFC.

Is correct, you can have quoted local parts and (I guess?) theoretically "foo"@mail and foo@mail should even be treated the same.

But practically this is a dead feature and probably should be treated as non existing.

AFIK `[<ip-address]` mails are used by some old data centers for delivering automatic generated "error" mails from unix server in a way which doesn't break when DNS is down.

Also interestingly the `[..]` syntax has a generic extension hook, and that hook allows usage of @ characters. So technically a `foo@[custom:@@@@@@@@]` is a valid mail address, just no one knows how to deliver it ;). (And `custom` must be registered with IANA, theoretically).

[mjmas]

From my reading through the RFCs a few months ago the message and smtp envelope also have different rules for addresses, and the message allows the local-part to contain whitespace but the envelope doesn't.

[dathinab]

both allow it but only if you use quoted text AFIK

through the message does allow an additional display name (like `display name <email>`) which has it's own rules.