Alleviate your free disk space problem and your email reception problem will go away. Anything else is just going to set you up for an even bigger mess when you completely fill the disks on the server computer.
Are you doing online backups of Exchange? I have a sneaking suspicion that you're not, and you're building up Information Store database transaction logs. If you don't know where to look, open the "Exchange Management Shell" and (assuming you left your mailbox store in the default "First Storage Group"):
get-storagegroup "First Storage Group" | format-table -Property LogFolderPath
You should only have ".LOG" files in that directory back to your last full online backup. If you're seeing a large number of ".LOG" files in that directory and, particularly if they're old, then you're probably not performing full online backups.
The "right solution" to that problem is to perform an online full backup. Anything else (switching the database logging mode to circular, performing a rebuild on the information store databases, etc) is a "hack" and isn't solving the problem in the way that Microsoft recommends.
Your WSUS directory is awfully big, but depending on the products and languages you have selected it's perfectly possible that it would be that big.
It sounds, to me, like you just need more disk. I'm assuming the rest of your space usage on the disk is in shared folders / files. Is there something there you can remove / relocate temporarily?
You could move the queue database to another disk. You say you've disabled back pressure, but you're still not receiving email. Double-check your procedure to disable back pressure and see that you really have disabled it and restarted the Transport service.