This is very common bounced back email error message when sending email is: 550 Mailbox quota exceeded. This means that the recipient was not able to accept the message because their inbox was full.
Q :How to Correct it ?
A : Normally the quota is space which we assigned during the account creation. You have full control over your accounts and manage it. If any email address has used the allowed quota, there are a few ways to correct this. First, you could simply delete some of your emails with heavy attachments, sent mail , empty trash. Deleting emails in your email client will remove them from the server and lower the space used depends the mode you are using.
You can simply increase the quota for the account. To increase the quota of email email account, first log into your cPanel.
1. Next, Check and click to the Email Accounts section:
2. Click on Show Disk Space Used and If any of email accounts are over quota, they will show in red as shown below:
3. To Increase the quota click on Change Quota and increase it under action.
4. Now once you increase the quota then you will see its not showing any red color for the account.
Account is now Good To Go