Cannot set up IMAP account via Outlook 365 on Window VM / Credentials OK with Outlook 365 on host

[camner]

I use Fastmail as my email provider, which requires a manual setup in Outlook.

When I use Outlook 365 on my host Mac, I can establish a connection with Fastmail.  I initiate setting up a new account, enter my Fastmail email address, specify I want to set it up manually, enter my app-specific password, and Outlook correctly sets up a new IMAP account.

On the guest Windows 10 machine I go through the same steps, but Outlook fails to create the account, terminating with the error message "Something went wrong and Outlook couldn't set up the account."

Since it works on the host, clearly it isn't because my credentials are invalid.

I definitely have a working network connection, because I can surf with a browser, connect to Windows Update, etc.

Any ideas about why Outlook 365 run on a Windows 10 guest wouldn't set up an IMAP account?

[Technogeezer]

Since you seem to have network connectivity, I would first see if Fastmail can be accessed via a web browser in the VM.  

I then would treat the problem as if this is a physical Windows 10 system and follow whatever troubleshooting steps Microsoft and Fastmail recommend when you experience connection issues on Outlook for Windows. 

Outlook for Mac and Outlook for Windows are not the same code or use the same OS “plumbing” under the hood. It’s possible that one will work and the other won’t because of those differences.