Plus Sign in Email Addresses

Plus sign in email addresses

The + has a very useful meaning in almost all mailers: stuff after it
is ignored. Ie, xyzzy@example.com and xyzzy+foo@example.com are
basically treated as the same email address by example.com’s
mailer. Both addresses deliver to the mailbox owned by xyzzy. This has
been a feature of sendmail for at least 10 years. It works in Postfix
and qmail, too.

The plus sign makes it easier to track how your email address is
spread around. If I ever get email to nelson+cheapviagra@monkey.org, I
know who to blame. It’s also helpful if you’re debugging webapps and
need to create email-keyed accounts.

From here