You get an returned email error message
Subject: Mail delivery failed: returning message to sender
'mailbox is full: retry timeout exceeded'
This means your mailbox is full.
What you need to do is to either clear out some of the mail in your mailbox or increase your quota.
To increase your quota, login to your Cpanel control panel,
under 'Mail Accounts' --> 'Manage/Add/Remove Accounts' click 'Show Disk Space Used'
click 'Change Quota' beside the account that is maxed out (eg, 10megs of 10megs used) and adjust your quota.
Possible you are leaving a copy of your mail in the server.
Possible if you are using a mail program like Outlook or Thunderbird etc that you have it not programmed to leave a copy on the server. Please check in the advanced settings and make sure 'Leave copy on the server' is not check marked. Once you remove the check mark it will remove the mail off the server and your disk space will be fine again.