libsrs2 is the next generation SRS library. SPF
verifies that the Sender address of an email
message matches (according to some policy) the
client IP address that submitted it. When a
message is forwarded, the sender address must be
rewritten to comply with SPF policy. The Sender
Rewriting Scheme, or SRS, provides a standard for
this rewriting that is not vulnerable to attacks
by spammers, is easy to parse by common mail
filters, and handles multiple hops neatly and
safely.
