smtp - Cannot connect to server from development machine. Works fine on prod.

I am using m06.internetmailserver.net and .NET Core, Syetem.Net.Mail.
The code works fine when running on the winhost server (i.e. the production web site) but when running on my local (development) machine it errors out when trying to connect to the smtp server. The error message is "A connection attempt failed because the connected party did not properly respond after a period or time, or established connection failed because connected host has failed to respond."
Since all works fine on the live web site I would guess the issue is not in the code.
My workstation is on a domain from work. When I test and get this error I am not at work though. I have a VPN installed which I use when I telecommute but it is not turned on when I test this site and get the described error. I hear that it may be some network issue, firewall, VPN, ISP, etc. but not having enough knowledge in that area I don't know what to check/test. Does anybody have some specific, clear advice?