When eM Client uses its Automatic setup for email accounts, it uses your mail server's Autodiscover settings to get the correct settings for all available services for your email account.
But it also checks some other details before, and even after if no autodiscover file is found.

The order in which eM Client check for these settings is as follows:

   1. Registry check - first eM Client checks your system registry to see if you have any overrides set for your account - if so, these settings are used.

   2. Service Connection Points (SCPs) or Active Directory for on-premise - some services publish information about their existence by creating SCPs, which client application can use for connection.

   3. SRV records in Domain Name System (DNS) - these identify computers hosting specific services and their ports, which are then used for the account setup.

   4. Office 365 - check if the domain and email are stored on a paid Microsoft 365 server, then use Office 365 setup (Exchange).

   5. Guess autodiscover address based on the domain name (https://your-domain/autodiscover/autodiscover.xml, https://autodiscover.your-domain/..., ...)

   6. Guess IMAP/POP and SMTP settings - try standard/common host names and port number until the mail server responds.

If all of these fail the program will ask you to use manual setup instead under Mail > Other.