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One of the worst e-mail outages you can have is when you get on a spam "blacklist." When this happens, you'll get seemingly random complaints from users about their e-mail not being received, which is because the recipient subscribes to the blacklist, and won't accept mail from your domain because you are known to send spam.
This is no fun to troubleshoot, and once you figure it out, it can be a long and arduous process to get off the list, since the blacklist maintainers are only interested in punishing people and relish the pain they're putting you through. So, the key to staying off blacklists is making sure your e-mail system doesn't turn into an open relay.
One very good way to do this is to test your system. You can do that by going to the following Website and entering your domain.
This will make several types of attempts to hack your system and use it to send an e-mail through. Hopefully, it will show you all the methods were unsuccessful, meaning your system is in good shape. Obviously, if one of the methods works, you will need to take steps to close it. This could mean reconfiguring your system, patching it, or perhaps even a major upgrade, depending on the vulnerability.
One other thing you should test is your systems management. You want to make sure you can find all these unsuccessful attempts in the log files. If you have the system set up to notify you, or if you have intrusion detection facilities, see if those catch the attempts as well. It's important for you to know what your systems catch and what they don't catch.
Finally, I recommend you run this test periodically -- perhaps once every quarter as part of a health-checking process