SymptomsSMTP servers reject incoming mail to a domain <domain> with the following response:
550 5.7.1 Unable to relay for <domain>
POA background task(s) "Unregister SMTP event sink(s) for domain" fails with whatever error.
CauseStarting from POA 2.6 SMTPEventSink is used for Exchange 2003 and there is a file named Domains.dat which contains a list of all hosted domains.
If this file is edited manually or restored from a backup, it can be out of date and some recently registered domains do not receive mail.
In case of a load-balanced SMTP cluster the problem can be intermittent, due to diffrent Domains.dat files are placed on cluster members.
ResolutionYou need to restore the file on all SMTP servers registered in POA. It could be done from the Provider CP:
1. Go to Top > Deployment Director > Server Manager > Hardware Nodes > <smtpserver> > Applications > ExchangeSMTP > SMTP configuration
2. Press Restore service configuration file button.
3. Go to Task Manager Top > System Director > Task Manager > Background Tasks
4. Wait for all tasks with name *SMTP* are processed.
Repeat steps 1-4 for each SMTP service installed in the system.
After the file is restored you can verify that the new version is up to date (contains all hosted domains). File is usually located in SMTPEventSink folder on your system drive on each SMTP server.