I've successfully set up mail server with relaying through my provider's smtp server with below 2 tutorials:
So now I'm able to send mails using smtp.xy.com:25 (after authentication), as outbounf port 25 is allowed by ISP.
I have dynamic IP so I use dyndns...I guess port 25 inbound is not allowed by provider. Also I use one dynamic dns name "02.dyn.com" set in field "mx record" for 01.dyn.com, as, although I have a paid account, mx record is not covered in it...
But how could I RECEIVE mails from gmail, for example?