Ask HN: Why does adding '+FB' to an email still deliver to the original address?

[el_hacker]

If you send an email to the address ‘[e-mail]’, which doesn’t exist, the email will still be delivered to the original address ‘[e-mail]’. Why and how is that possible? I think for this to work, the email address should first be used on a Facebook account.”

[curious_curios]

Everything after the plus is considered a sub address and is just an arbitrary tag. You can include any text there and it’ll get delivered to the primary address. It’s useful for filtering emails, and is part of the email spec.

[joezydeco]

However, a lot of websites aren't programmed to the full spec. I've seen many sites reject the use of '+' on the left side.

[PaulHoule]

Some web sites don't want you create accounts like

  [e-mail]
  [e-mail]
  [e-mail]

when it comes to making an army of sock puppets the biggest hassle is usually doing the email verification for all the accounts. The "+" trick greatly simplifies this though if somebody is looking for trouble they sure will find it. Of course if you control your own domain you can use a wildcard but if you add 1000 users to a site from the same domain that is obvious too.

[PaulHoule]

I understand that the email server has digression on what to do with the local part of the email address. For instance a mail server could take mail to [e-mail] and put it in the "wildcard" box.

I was a qmail fanatic circa 2001 and back then qmail was often configured to put emails to "[e-mail]" to the unix account you and also send "you-*@yourdomain.com" to the you account (anything that starts with "you-")

Of course you can use this to tag email address so if you subscribe to Facebook using "[e-mail]" then emails from Facebook are sent to "hn".

I think the people at gmail thought this was a good idea but that it was better use + instead of - so there you go.

[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.

[nhggfu]

don't forget the dot-hack also [e-mail] = [e-mail] = [e-mail] etc

[solardev]

This also depends on the email provider. It works on Gmail but not all email hosts.

[lofaszvanitt]

postfix:

recipient_delimiter = +