Solve E-mail Space Limitations in Plesk
From Lunarpages Web Hosting Wiki
When you are hosting on a Windows-powered hosting plan and you are not able to get e-mail via the POP3 or webmail methods, it could be because you have exceeded your space limits. This can easily be fixed by logging into your mail account and deleting un-needed mail or raising the disk space limitations on that particular e-mail account.
Please Note: The shared Windows hosting plans (not VPS or Dedicated) have an e-mail storage limit of 500MB which can not be changed.
Here is how you could do that from within Plesk:
1. Login to Plesk
2. Select Desired domain
3. Select Mail(Remote)
4. Select email account (You will be under user tools)
5. Select Mail BOX( Mail box Management)
6. Make sure Mail box is selected
7. Select Radio button and enter the size in kilobytes
8. Click the, "OK" button
This should help you combat any issues with not being able to send or receive new mail on any Lunarpages Windows hosting powered plan.