[dfawcus]

Whereas email more or less lasts forever (mailbox contents), and has to be backwards compatible with older versions back to (at least) RFC 821/822, or those before. It also allows almost any character (when escaped at 821 level) in the host or domain part (domain names allow any byte value).

So a Internet email address match pattern has to be: "..*@..*", anything else can reject otherwise valid addresses.

That however does not account for earlier source routed addresses, not the old style UUCP bang paths. However those can probably be ignored for newly generated email.

I regularly use an email address with a "+" in the host part. When I used qmail, I often used addresses like: "foo-a/b-bar-tat@DOMAIN". Mainly for auto filtering received messages from mailing lists.