I have a tip for those using Google Mail. I've noticed that lately I haven't been receiving much mail, but didn't know why. I receive my Google mail not on the Google site, but through my mail program, so I had no idea that I had used up my allotted amount of disk space. No warning; no auto-email – nothing. Most of my mail was bouncing. In Google mail, your mail is archived not deleted and quite honestly, I forgot about it. The fix was to delete my archived mail (all 160,000+ of them and then empty the trash.
If you have Google mail, you may want to check it now so that you will not be left with a mess as I did.