Thanks for your response. My apologies, I posted about this problem on another forum as well, and forgot to update this one.
As it turns out, our firewall wasn't blocking any ports, it was just not allowing other SMTP traffic than specified, and nothing was specified.
Anyway my code was good, including TLS, so the question remains, how come Outlook was not blocked by our firewall, when my code was.
My advice is free, and you may get what you paid for.
modified 1-Mar-18 4:12am.