Only one mail exchanger, on one address
NOTEWhat it means
example.com has a single MX host and it resolves to a single address. While that one machine is unreachable, senders queue your mail and eventually return it. a maintenance window turns into lost mail.
How to fix it
Add a second mail exchanger at a higher priority number. Most providers include one at no extra cost.
Check your own domain
This page describes the condition. To find out whether your domain has it, run the MX test or the full checkup, which examines every record at once.
Understand it properly
This page covers one condition. For the whole picture, Why mail lands in spam, in the order worth checking, or start at the Health Library.
Where this comes from
We judge this against RFC 5321 section 5.1. Every finding on this site cites the specification behind it so you can check the work rather than take our word for it.
Other MX conditions
MX_SINGLE_POINT_OF_FAILURE
Every fix on this site is yours to implement. If you would rather someone did it, I take this work directly.