Just some advise....I would check your mail folders first. More often than you can imagine, when your space fills up for no visible reason, the default mail account is the culprit, as it usually contains all mail errors, spam, etc, that is sent to your domain, without a valid email address.

Example...
Someone sends spam to your domain at user1@domain.com, but user1@domain.com doesn't exist on your account. It will get thrown into the default address.