You can sometimes receive a bounce mail after you send a mail to somebody stating that the Recipient's mailbox is full. The cause and solution for it is explained below:
Hi. This is the qmail-send program at plesk01.diadem-tech.com.
I'm afraid I wasn't able to deliver your message to the following addresses.
This is a permanent error; I've given up. Sorry it didn't work out.
Recipient's mailbox is full, message returned to sender.
This error is often encountered when the mail id to which you are sending mail has crossed its user quota limit and hence the mails sent to that id is bounced.
The mails in the recipient mailbox has to be deleted or downloaded via a mail client so that the mailbox size comes under the allotted quota limit and after which it can receive new mails.