To determine if your ISP is blocking SMTP port:
Open a command prompt by going to "Start" -> "Run" and typing "cmd" or "command" (depending on your version of windows).
At the command prompt, type:
If that works, then type this:
telnet mail.yourdomain.com 25
If you receive, "connection timed out", then your ISP IS blocking port 25 the SMTP port.
If you receive "220-titania.hostingmadeeasy.com ESMTP Exim 4.24 #1", then your OKAY.
If you received "connection timed out" then contact your ISP for more details about your block and how you can send e-mail using their mail servers.