proxsmtp - 1.10-2.1+b2 main

ProxSMTP is a flexible tool that allows you to reject, change or log email
based on arbitrary critera. It accepts SMTP connections and forwards the SMTP
commands and responses to another SMTP server. The 'DATA' email body is
intercepted and filtered before forwarding.
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You need to be able to write the filtering scripts that integrate it with your
particular needs. If you're looking for something that does virus filtering,
take a look at ClamSMTP which behaves similarly and uses a similar code base.
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You do not need a local mail-transport-agent installed, since this server can
forward to a remote one.