Plus signs ("+") in email addresses

The use of a plus sign ("+") in an email address can be very useful. Gmail, as well as two common mail server systems, sendmail and CommuniGate Pro, will deliver mail addressed to "user+tag@mumble.dom" as though it were addressed to "user@mumble.dom." The "+tag" portion of the email address can then be used both for filing mail and for screening it.

Filing is the simplest application to take advantage of plus signs.

Screening is a useful way of avoiding spam and/or other unwanted correspondence.

Email addresses with a "+" are VALID (Anders Jacobsen's been using "+" in email addresses with qmail since 2002)
RFC 822 - Standard for the format of ARPA Internet text messages (referred to in above blog entry)
RFC 2822 - Internet Message Format (obsoletes RFC822, but supports legality of "+" in email addresses)
Characters in the local part of a mail address (A table of characters which confirms "+" is allowed in email addresses)
Email Addressing FAQ (How to use user+box@host addresses)
Can I receive messages at a different Gmail address? (The Gmail Help Center explains how to use plus sign in place of traditional aliases)
E-mail with a "Plus" (Jim Leous describes the advantages of Penn State's new support of this feature)

This page last updated November 3, 2010.