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MailScanner now includes support for SpamAssassin. This is "a virus scanner for e-mail designed for use on e-mail gateways. It can also detect a large proportion of unsolicited commercial e-mail (spam) passing through it." MailScanner does an excellent job with file names and types, and can be set to quarantine encrypted .ZIP files and apply normal filename rules and virus scanning inside unencrypted ones.
OpenProtect is a complete package which includes MailScanner for intergration with MTAs, spamassassin, clamav and it can be installed within 5-10 minutes and no need to manually edit any configuration files or settings. [OpenProtect|http://openprotect.com] supports postfix, sendmail, qmail and exim. See [http://wiki.apache.org/spamassassin/OpenProtect] or [http://openprotect.com] or [http://openprotect.sf.net] for more details.
Another option is to use a filtering SMTP proxy such as spampd. It is typically used to filter the mail between an external-facing MTA instance/relay, and an "internal" MTA instance. http://www.worlddesign.com/index.cfm/rd/mta/spampd.htm
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