When the user reached the mail quota, for example 100 MB, the user can't recieve any more mail, but the real problem is the user can't delete mail and free up space in their mailbox thru a WebMail login.
I have 100's of webmail-only user accounts, and the only thing I can do is raise the quota of their inbox.
What happends when you try to delete mail it goes to trash first, and when your quota is full you can't move mail! And when you can't move it you can't trash it!
in SquirrelMail go to options, then Folder Preferences then on special folder options set trash folder to [do not use trash], hit submit and you should be able to delete unwanted mail. If you still want to use the trash folder just change it back as it was before.
What i meant is that the user who is overquota needs to go login to his webmail account and access the options link at the top of the interface. The downside is that this must be done for each user that is overquota.