The most common problem for this issue is a full mailbox on the server.
You can log into the Plesk Control Panel to view your mailboxes, it will indicate when your mailbox is full.
You can also login to the Webmail Interface with your email address and password to view the email located on the server, from here you can also delete unwanted email and clean out possible main in the Junk/Send folder.
We would also advice that you receive your Email Client settings.
If you are using POP3, please ensure that you do not leave email on the server for too long before deleting it.
If you make use of IMAP then you need to manage your folder properly since all the email will be stored on the server and will only be deleted when you delete in from your email client.