I keep getting the following message "There is not enough disk space to download new messages..." I have deleted & compacted.

I keep getting the following warning message: "There is not enough disk space to download new messages. Try deleting old mail, emptying the Trash folder, and compacting your mail folders, and then try again."

I have deleted a substantial number of emails, compacted folders, and restarted my computer (several times) and I'm still getting the same warning. I cannot receive any new emails.