Squirrelmail says I'm over quota! But I can't delete messages because I'm over
quota! Help!

Solution:

You are seeing this problem because the large email put you either over your
available quota or close to over your available quota.

When webmail deletes an email message, by default it moves it to your trash
folder, by first copying the message to the new folder, then deleting the
original message. This copy operation is failing because Webmail doesn't have
enough room to copy the message, so it never gets to the second step of
actually deleting it. The solution to this is to temporarily turn off your
trash folder, so that when you delete a message, it is immediately gone.

To do this:

1) Log into Webmail and click on the Options link.
2) Click on the Folder Preferences link.
3) At the top under Special Folder Options, you'll need to change the Trash
Folder option to "[Do not use trash]"
4) Click the Submit button at the bottom to save your changes.

Once this is complete, you should be able to delete messages again. Once the
large message is gone or you're back under quota, just follow the steps above
again and change the Trash Folder option back to "Inbox.Trash"