Hence an email address/mailbox/addr-spec is "local-part@domain"; "local-part" is composed of one or more of 'word' and periods; "word" can be an "atom" which can include anything except "specials", control characters or blank/space; and specials (the *only* printable ASCII characters [other than space, if you call space "printable"] *excluded* from being a valid "local-part") are: ()<>@,;:\".[] Therefore by the official standard for email on the internet, the plus sign is as much a legal character in the local-part of an email address as "a" or "_" or "-" or most any other symbol you see on the main part of a standard keyboard.

There's a common design flaw on many many websites that require an email address to register; most recently I came across this bug on CNet's download.com site: for some reason they don't accept me when I try to register an email address containing a "+", and they then send me back saying my address is invalid. It isn't!

"+" is a completely valid character in an email address; as defined by the internet messaging standard published in 1982(!) RFC 822 (page 8 & 9)... Any website claiming anything else is wrong by definition, plus they are prohibiting me and many fellow anti-spam activists from tracking where inbound spam comes from: