RFC 1855 defines e-mail signatures as text following the character sequence "-- " (two
ASCII dashes and an ASCII space) on a line by itself. Everything following this sequence is considered part of the
signature and is not necessary to be included in replies.
A common mistake among people who attempt to get this right is to neglect the space (ASCII 32) character,
as it is not usually noticeable when attempting to mimick the sequence. The space helps reduce the chance of the sequence mistakenly appearing earlier in the message (since it does not serve a useful purpose otherwise, and is visually identical to two dashes).
E-mail signatures should be limitted to no more than four lines in length and provide information about
the message sender, such as the sender's name and alternative contact methods. Often e-mail signatures may include a
Web address, information about the sender's
PGP key, or--space permitting--a
witty line.
It is considered very rude to include very long signatures and to not remove them prior to sending a reply.