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That's part of the RFC email address spec. Ignoring .'s isn't, AFAIR.

Nope, the + trick is not standardized. The RFCs only specify that everything before the @ character is specific to the recipient system and Should Not Be Messed With; even case-folding that part of an e-mail address is a no-no, since the recipient system might distinguish case, and the sender has no way of knowing.

The + trick arose later, as a feature of certain mail recipient programs like Sendmail and Postfix, and most of these programs allow you to change from + to any character you like.

Subaddressing or "plus addressing" is a standard defined in RFC5223.

Note that RFC5233 doesn't actually standardize the delimiter. It uses "+" "#" and "--" in examples.

All RFC5233 does is specify how sieve (a protocol for specifying server-side email filters) can describe filtering on subaddresses. It does not confer any actual interpretation on email addresses, especially since many email providers don't actually use sieve for their email filters.

Which RFC spec? Not 2822?

RFC5223 defined subaddressing.

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