When one of your users complains that your mail server “doesn’t work”, 90% of the times it’s his own fault, and 10% of the times it’s someone else’s fault. You’ll notice that 90% + 10% makes 100% of the cases. Actual problems on the server do happen sometimes, but the chances that your users notice before you do are basically zero.
You guessed it, today I got one too much of such “the server doesn’t work” complaints.
Make that 91%. “Bad destination host ‘DNS Hard Error looking up xxxxx.it (MX): NXDomain’” does not mean there’s a problem on the server when one writes the domain name incorrectly.