It was due to the other mail servers missing the defaultdomainhostname and mail.[DOMAIN_NAME] rows. I've manually entered them and it is working now.
I'm running into an issue trying to import a mail domain. Nothing is happening in the GUI and there is an error message in the Event Viewer:
[11/25/2024 1:26:02 PM] ERROR: 'SmarterMail 100.x +' DomainAliasExists
System.Exception: Could not get the list of mail domain aliases ---> System.NullReferenceException: Object reference not set to an instance of an object.
at SolidCP.Providers.Mail.SmarterMail100.GetDomainAliases(String domainName)
--- End of inner exception stack trace ---
at SolidCP.Providers.Mail.SmarterMail100.GetDomainAliases(String domainName)
at SolidCP.Server.MailServer.GetDomainAliases(String domainName)