When configuring the MX records for your domain there are a few simple guidelines to follow. The lower the number the higher the priority. For example if your MX record priorities are 10 20 and 30 the MX 10 should be the first mail server that is tried when someone is trying to deliver email to your domain. Common MX priorities numbering schemes include going by 10's or 100's such as 10 20 30 or 100 200 300. While the service still exists I do not recommend any sort of backup-mx or mail spooling service since those mail servers are targeted by spammers and they generally have weaker anti-spam mechanisms in place. For the technically curious the largest MX priority that you can set is 65535 since the value is a 16-bit field in DNS. If you try to use a number greater than 65535 you run the risk of unexpected results and even causing your DNS server to crash or fail to load your zone file.
