That's not very nice language, is it?
I mean, that my domain might be a problem. Hmmph.

Well, let's read on in your email - what is the problem?
Usually it refers to a disk space overage.

Example:

Limit has been exceeded on domain mydomain.com' owned by client Me (Contact name First Last):
Disk space usage: 1203.16 M
Disk space limit: 1200.00 M
Traffic: 1260.49 M
Traffic limit: 10.00 G

This is usually caused by a buildup of email on the server to the extent that it has now pushed over the limits set for your account.

POP - if you have your email configured for POP, you should also have it set to remove email from the server after it is downloaded. These are settings within your email client (program) and not something we can do for you. If you want to store email on the server and want to purchase more domain space, please contact us for a quote.

IMAP - if you have your email configured for IMAP, then you may accumulated emails to the extent that you have now exceeded your allocated space limit. If you want to purchase more domain space, please contact us for a quote.

Other possiblities for this message:

You have exceeded bandwith allocation. Congratulations on having a lot of traffic to your site! If you want to purchase more bandwidth, please contact us for a quote.