The problem with "+" isn't the email side.

The problem with "+" is dumbass Javascript developers who use broken regexes to "validate" an email address.

right. that's very frustrating. just a few days ago i had to create another email alias because of it.

+ was always a legal character, right from the first SMTP RFC 822

My email for one of my CC accounts is like ...+banksucks@gmail.com because it allows plus-addressing, but doesn't like my domain.

In other words, don't worry, they'll find a way to block your email one way or another. :P

It's gmail in the sense that they deliver you+[whatever]@gmail.com to you@gmail.com

By default it's just a valid character.

nope, exactly this feature you describe is what i am referring to. it's not a gmail invention. not by a long shot.

Not a gmail invention perhaps, but also not per RFC. That some use it to mean something special is not in the RFC. Actually, a significant number of SMTP servers don't even implement the required parts of the related RFCs, let alone fancy things like plus handling.

it is not in the SMTP RFC because it is a purely local matter that does not affect email routing.

there are a number of servers that support it. wikipedia lists some if them:

https://en.wikipedia.org/wiki/Email_address#Sub-addressing

Agreed, most of email servers and services are broken per RFC. I've blogged about this over and over again.

I think it originated with CMU's email system (the use of the "+" sign specifically).

You're right. Originally the + sign in an email address was an indicator to the Andrew Message System's delivery agent to process the email in an extensible way. The syntax was +<keyword>+<args>. As an example. you could use "user+dir-insert+misc" to route the message to the "misc" directory in the user's mailbox structure. An unknown keyword would just get ignored and the mail delivered as usual, giving the behavior as used today.

TIL! Thanks, was not aware of that.

As stated by others, + addressing is not gmail specific. One thing that gmail does however is allowing you to add (or remove) arbitrary dots in your mail-address, and these are stripped out / all end up in the same mailbox.