Getting over usage e-mail notifications by Plesk:
The `<domain.tld>` subscription, owned by user `<username>`, is approaching or already exceeded the resource usage limits: Total MySQL databases quota: 5 GB limit / 882.7 MB (17%) used / 4237.3 MB (83%) remaining
when actually there is no overusage.
This issue has been considered as software issue #PPPM-2740. It was fixed in Plesk 12 microupdate #44.
For Plesk 12 apply the latest Plesk microupdates.
As a workaround replace the file
%plesk_dir\%admin\plib\Limits\Limit\DatabaseDiskSpaceAbstract.php with one in pppm-2740.zip archive.