[Talk] Username + detail addresses

[Peter Jeremy]

Sendmail and some other MTAs support the use of subsidiary information
in e-mail addresses so that a mail sent to "user+detail at local.host"
will be delivered to "user at local.host" and the user can then use the
'detail' for anything they like.

Can anyone point me to any standards where this is defined?  Both
RFC-822 and RFC-2822 just describe the local-part of the address
(everything before the '@') as domain dependent.

Peter