You can run a simple test to see if your ISP is blocking port 25. Open a command prompt for windows or Terminal.app for OS X and type in the commands below.telnet yourdomain.com 25
If your ISP is blocking port 25 you will not get a response back. However if it isn't blocking port 25 you will see an output similar to the one below.[user@host:~]$ telnet yourdomain.com 25
Connected to yourdomain.com.
Escape character is '^]'.
220-host.domain.com ESMTP Exim 4.69 #1 Mon, 07 Jul 2008 00:22:59 -0600
220-We do not authorize the use of this system to transport unsolicited,
220 and/or bulk e-mail.
Please keep in mind that you should be using a secure (SSL) connections for Outgoing SMTP and Incoming IMAP or POP3.
Outgoing SMTP Port 587, Incoming IMAP Port 143 and for Incoming POP3 port 110.