Window hosting and Linux hosting step checking for email issue:
Window hosting
1. Login to MEAdmin / Smarter Mail / HELM mail function
2. Check for the email account quota size that having this error problem
3. Problem occur normally is the email quota size had been reach the limit
4. How to solve: Increase the quota of the email account.
Linux hosting
1. Login to CPanel
2. Click on Mail
3. Then click ‘Manage/Add/Remove Accounts’
4. There is a button name ‘Show Disk Space Used’, click on it.
5. Then check whether the email quota is reach.
6. If the quota already reaches, click on ‘Change Quota’ to increase the quota of the email account.