Mail is just the way Linux tells you that it has reported an error, and made a log file that it stores in the “mail” folder. It’s not anything to do with email or anything.
Unless you know how to diagnose it, you can probably ignore it.
Here’s a reference describing it.
If in future you ever get the same dialogue, and it’s annoying you, you can remove it with these instructions.
That said, it could be useful to read the mail, to find out exactly what’s causing problems. There’s instructions showing you how to do that below as well.