[koutetsu]

If I'm not mistaken, it's a Gamil specific feature. Any combination of words or numbers between + and @ is seen as not part of your email address but can be used by filter received emails.

See this link for more information: https://gmail.googleblog.com/2008/03/2-hidden-ways-to-get-mo...

[IcePic]

It's not gmail specific, it's standardized as a "sub-address" and not part of the username of the email. smtp has worked like this since the 90s at least.

[solardev]

Do you have a cite for this? AFAIK "plus addressing" always been an optional implementation done by certain email providers themselves, but wasn't an actual part of the standards.