I/O 997 operation in progress error The I/O 997 operation in progress error occurs for some unknown reason. Usually the 997 Overlapped I/O Operation in Progress error is caused by the following factors:

1. Incorrect directory address setting in main SPOOL directory.
2. Incorrect directory for the virtual domain SPOOL directory.
3. The default Windows NT SMTP service is running.
4. Incorrect permissions for the services running.
5. Using DHCP instead of STATIC IP address.
6. DNS IP address is not specified or not valid on the mailsite host machine.

If running MailSite 4.x (non-DataCenter), ensure that for item 4, services are running with system permissions. To verify, check under administrative services in the control panel.

If running MailSite 5.x (SE/LE), ensure that for item 4, if MailSite is running in "simple" mode, that the services are running with system permissions. If MailSite is running in "secure" mode, ensure that the services are running under the MailSite-User account.

Additionally for NT 4.0 only, you may need to do the following:

1. Uninstall mailsite.
2. Run reg clean, to clean up the registry.
3. Reboot.
4. Reinstall Mailsite

If you fail to reboot, the error will occur again.