You can run a simple test to see if your ISP is blocking port 25.

Open terminal and type below command.( for windows: Start >> Run >> Type ‘cmd’)

telnet domain.com 25

Replace domain.com with your domain name.

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.

stefin@fwhwin:~$ telnet flashwebhost.com 25
Trying 119.81.4.114...
Connected to flashwebhost.com.

Escape character is '^]'.
220-server20.hosthat.com ESMTP Exim 4.82 #2 Thu, 04 Sep 2014 05:34:20 +0000
220-We do not authorize the use of this system to transport unsolicited,
220 and/or bulk e-mail.