[MatekCopatek]

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

By default it's just a valid character.

[em-bee]

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

[sethammons]

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.

[wiml]

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

[MatekCopatek]

TIL! Thanks, was not aware of that.

[sigio]

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.