On a recent blog post I had listed a couple of handy free admin tools in the vein of DNSstuff aka DNSreport. Here is another great freebie for checking some basic MX stuff. Find out if your mail server is a relay, if you have a reverse record, if you are on a blacklist, how fast the respond time is, and some other very useful information. Plus you can’t beat the FREE cost.
The earlier post is here: https://duitwithsbs.wordpress.com/2008/06/04/some-handy-web-tools-for-the-admin/