I can't access my STMP server.

A number of people have asked me, this kind of question:
"I bought a domain name and signed up a web hosting company to use an email with my own domain.
The web hosting company gave a SMTP server address.
But I can't access that SMTP server from my email client."
In the most cases, your ISP blocked you to use the third party SMTP server rather than the one your ISP provided.
You only need to fill your ISP SMTP server address.
Then that works!

Is C# MS-only?

No, No, No. There is a open-source version, Mono. They can run in a lot of different os.