Cause: Mail domain had Insufficient disk space.
Step1: Access SmarterMail with admin user to check the disk usage.
Step2: At SmarterMail, Navigate to Reports->Expand the System Summary reports->Now Traffic Reports->Disk usage
Step3: From the disk usage, we have found that it was entirely utilized.
Step4: We removed the unwanted emails and data to release some disk space and at the end the issue was fixed.