The mailbox quota is the limit on the size each mailbox can reach. The disk space quota is the total amount of disk space, including email, that your account has. When an account reaches the limit of its disk space quota no more files may be added to the site.
In Plesk, each mailbox can grow up to the total size of the mailbox quota, even if it exceeds the site disk space quota. No additional files will be allowed to be FTPed to the site until the disk space quota is below its limit.
For example, assume a site has three mailboxes with a mailbox quota of 10 MB. The site has a disk space quota of 25 MB. If the site has:
Web Site: 15 MB
Mailbox1: 3 MB
Mailbox2: 1 MB
Mailbox3: 10 MB,
then Mailbox1 and Mailbox2 will still be able to receive mail, while Mailbox3 will not be able to receive mail. Since the total disk space used is 29 MB, the site user will not be able to upload more web files to the site. The client will receive an email once a day that this domain has exceeded its allocated disk space. If an account is consistently using more than the disk space quota due to extra email additional disk space charges will apply.