Where is the + in email addresses defined?

I know that email addresses, in the local part (before the @ sign), can include a +. And everything after the + can be used by the email system for filtering and other uses. But the email itself is delivered per the part before the + sign.

Example:

larry@foo.com         Delivered to larry mailbox on foo.com
jim@foo.com           Delivered to jim mailbox on foo.com
larry+1@foo.com       Also delivered to larry mailbox on foo.com. The +1 can be
                      used for filtering or something else by the recipient
larry+hi_guys@foo.com Also deliver开发者_JS百科ed to larry mailbox on foo.com

The above works on GMail, Yahoo and other systems.

Question: Where exactly is it defined in the RFCs? I looked but couldn't find it.

The + isn't defined in the RFC for mail addressing, RFC 822. It is an implementation by some MTAs in an effort to filter incoming messages to a single mailbox, but the current RFC doesn't' support exposing mailbox structure/filters to senders.

It does leave the handling of the local portion of the address up to the MTA which is where these extensions have been promoted.

There is a proposed RFC, RFC 5233, for sub addressing that explains plus addressing in detail.

So there is ongoing work to standardize it, but that hasn't yet happened. I know not all MTAs use + addressing for this purpose.