Technical Requirements: Filters and Receivers

In any email system, there are normally components that correspond to the "mail transport" and also to "email servers". Whenever a client is ready to send a message, one or more mail transport agents (MTAs) will "transport" the message from the client to its destination server, where it is stored.

It is natural to view archiving, quarantining, and virus-filtering as MTA functions.

For archiving, before (or at the same time as) the MTA delivers the message to a destination server's message store, a copy of the message must be delivered to an archival "receiver", i.e., to an archival store. As well as retaining a definitive copy of the message, archival receivers and their accompanying archival software typically provide sophisticated search and correlation capabilities, allowing administrators to search within the archive for emails between certain people that contain a certain word, and so forth.

For virus filtering purposes, the MTA passes the message through a "filter" prior to delivering it to its destination server's store. If the filter detects a virus it can take appropriate action depending on the filters configuration, usually one of: i) bounce the message; ii) deliver the message with modified title and content to indicate it may contain a virus; or iii) divert the message to a "quarantine" account where a system administrator can decide whether or not it should be delivered.

In some archiving situations, both the delivery to an archival receiver and filtering is used. For instance, in a financial institution message archiving may be used to create a definitive record of all emails, but at the same time staff not permitted by regulation to communicate with one another may have their emails filtered to prevent unauthorized messages being passed.