Exchange generates this error when the available hard disk space on the server falls below 4 GB. Exchange pauses mail acceptance to protect database integrity.
Solution
- Free up disk space on the Exchange server (clear logs, temp files, etc.).
- Check the BADMAIL folder in the POPcon program directory — move any
.msgfiles to the PICKUP subfolder to retry delivery once space is available. - Consider using a monitoring tool (e.g. ServerPulse) to alert you before disk space runs critically low.