The MailEssentials Dashboard generates a 503 Service Unavailable Error when configured to run in IIS mode. The IIS Application pool crashes at the first request and stops itself.
This issue has been observed in IIS 7.5, running .NET 4.8, on Windows Server 2008R2 SP1 and it is caused by 32-bit Applications being enabled for the MailEssentials Application pool.
In IIS Manager, select the MailEssentials Application Pool, and under Advanced Settings, set "Enable 32-bit Applications" to false.
Open a command prompt as Administrator and run