If someone gets bounced mail messages when trying to send mail to your Plesk based mailbox, and that message look like following:

---

This is the mail system at host www.domain_name.com

I'm sorry to have to inform you that your message could not
be delivered to one or more recipients. It's attached below.

For further assistance, please send mail to <postmaster>

If you do so, please include this problem report. You can
delete your own text from the attached returned message.

                   The mail system

<username@domain_name.com>: Message can't be delivered

---

When you try to look out for maillog (located in /usr/local/psa/var/log/maillog) you can find insufficient info about possible cause. There is no error number, just message:

status=bounced (Message can't be delivered )

Cause for mentioned error is full username@domain_name.com mailbox. Please ensure that user has enough disk space for mails.

Quota for domain must be set via

Home ->   Domains  ->  domain_name.com ->  Resource usage

Quota for account can be checked via

Home ->   Domains  ->  domain_name.com ->  Mail Accounts  > username@domain_name.com