MailCorral is a robust, high-performance virus/spam filter and archive program that runs as part of sendmail (using the milter interface) to filter out viruses and spam from all mail delivered on the site running sendmail. It handles all currently known malicious attachment types plus attached and inline HTML. It uses built-in and plug-in classifiers to decide whether mail is spam or contains a virus. It can archive some or all of the messages it sees.

MailCorral can be installed as part of a sendmail installation and left running unattended. It will remove harmful viruses from email and notify recipients by inserting text into the processed message, as it is delivered. A backup copy of the unfiltered message is kept for a fixed length of time, just in case filtering renders it unusable and the recipient needs to see the original. End users may easily request delivery of any message, in its unaltered form, via an automated delivery robot. Virus classification can be handled by the built-in rules or an external classifier (e.g. ClamAV) can be seamlessly invoked.

In the most common mode of operation, spam is simply not delivered. Instead, it is redirected to a corral where it can be released at a later time. Spam identification is accomplished either through built-in fast path rules or a connection to the popular spam recognizer SpamAssassin.

If you elect to use MailCorral's optional, seamlessly integrated, sister spam handling package (SpamCorral), end users are automatically sent periodic summary messages which will allow them to release for delivery, only those pieces of spam that they actually want to see. Other delivery modes allow spam to be discarded or delivered with a tag that allows it to be separated by a recipient's mail reader. Regardless of how spam is handled, processing of it is completely unattended by your system administrators.

Message archiving can be handled in either of two ways. If you have your own archiver, MailCorral can send a copy of every message it sees, through regular email channels. Many archivers are set up to receive mail in this manner and archive it as it arrives. MailCorral goes further, though, in that it looks for bounceback messages from the archiver and silently deletes them, thereby solving a common problem with such archiving schemes, viz. the unwanted return of messages, meant for archiving, to the orignial sender, when the archiver is down or unavailable.

The second way that MailCorral handles archiving is to actually maintain the archive itself and write a copy of each message to it. The messages can be indexed in a MySQL database or simply written to the archive where they can be post-indexed by a standalone program. Either way, the list of recipients whose email is archived is fully configurable.