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.