The following step is for checking email issue for Window and Linux hosting:
1. Login to MEAdmin / Smarter Mail / HELM mail function /DotNetPanel 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. To resolve this issue, increase the quota of the email account.
1. Login to CPanel.
2. Click on Mail icon.
3. Then click "Manage/Add/Remove Accounts".
4. There is a button named "Show Disk Space Used", click on it.
5. Then check whether the email quota is reached.
6. If the quota already reaches, click on "Change Quota" to increase the quota of the email account.
Any further assistance may submit ticket at firstname.lastname@example.org