mrt is a software tool that can check an Internet e-mail server, or arbitrary groups of Internet e-mail servers, to see if they can be used by unauthorized parties to relay e-mail messages sent by those unauthorized parties to other Internet users who are not normally served by the e-mail server(s) in question.

Unlike the interactive Abuse.Net mail server tester, the mrt tester is a program that you must download and run on your own computers. Mrt also differs from the Abuse.net mail server tester in that it allows you to perform full end to end relaying tests on mail servers. Mrt will actually inject a message into each mail server being tested. You must then wait to see if these test messages actually come back to you. (The Abuse.Net tester only does a half-hearted job of mail server relay testing; It only checks to see if a mail server would be likely to accept a message for relaying, not whether it would actually relay that message to a destination address of your choosing.)

Another unique feature of mrt is that it can be used to scan large numbers of mail servers for relaying vulnerabilities at a high rate of speed. It does this by making use of multiple concurrent subprocesses, the number of which can be selected by the user. Mrt can check groups of mail servers located at specific individual IP addresses specified by the user, or alternatively, it can be used to check all servers within a specified range, or ranges of IP addresses for mail relaying vulnerabilities.