1. Open Thunderbird.
2. Go to your inbox and remove any unneeded email, particularly large messages.
3. You can sort the messages by size by clicking on the Size column header. Click the column header twice to sort by size in descending order. If the Size column is not visible, click the button above the vertical scroll-bar in the message pane and select Size from the drop down list.
4. Download any attachments you want to keep and delete those messages.
5. Do the same for other mail folders on ArtSci to keep your account size under quota.

To set Thunderbird not to save deleted message or sent mail:

1. Go to the Tools menu and select Accounts Settings.
2. Select Server Settings in the left menu listed under your ArtSci account.
3. Find the option When I delete a message: and select Remove it immediately from the drop-down menu
4. Check Empty Trash on Exit
5. Click OK to save your changes