how do I access smtp.mydomain to be able to send email as in gmail?

I am trying to access smtp.mydomain.com to be able to send emails from something@mydomain.

Basically, what I want to do is add ‘something@mydomain’ as a ‘send-from’ email in gmail. but to do that it asks me to log into smtp.mydomain. so how do i log in to smtp.mydomain.com? what is my username and password?