E-MailRelay is a simple SMTP proxy and store-and-forward message
transfer agent (MTA). When running as a proxy all e-mail messages
can be passed through a user-defined program, such as a spam filter,
which can drop, re-address or edit messages as they pass through. When
running as a store-and-forward MTA incoming messages are stored in a
local spool directory, and then forwarded to the next SMTP server on
request.
