This plugin is used to 'rescue' legitimate bounce messages that were generated in response to mail you really did send. List the MTA relays that your outbound mail is delivered through. If a bounce message is found, and it contains one of these hostnames in a 'Received' header, it will not be marked as a blowback virus-bounce.
The hostnames can be file-glob-style patterns, so
relay*.isp.com will work. pecifically,
? are allowed, but all other metacharacters are not.
Regular expressions are not used for security reasons.