Nowadays, most of the ISP’s are blocking the port 25 case that can be simply tested by establishing a telnet connection on port 25:
$ telnet dragos.fedorovici.com 25
telnet: Unable to connect to remote host: Connection timed out
If you see the above error message, your ISP is very likely blocking the ability to send mail through servers other than theirs. Over the last few years, more and more ISP’s have started to require their subscribers to use their SMTP server to send mail. This allows them to monitor spammers, and to reduce to the overwhelming amounts of spam and exploitation that occurs daily with email. You may want to contact your ISP to see if they are willing to work with you on this issue or try port 587 or 26 which may be opened.
When it comes to your server, if you are running Exim as the default MTA on your server, you can bypass this block from different ISP’s, by opening an alternate port for Exim. and allowing your clients to use this port for outgoing SMTP. For this you will need to edit the configuration file of Exim (usually /etc/exim.conf) and add a similar line to the configuration file:
daemon_smtp_ports = 25 : 587
Restart the mail server and you should be able to bypass the restriction added by your ISP:
$ telnet dragos.fedorovici.com 587
Connected to dragos.fedorovici.com.
Escape character is ‘^]’.
220-torch.lunarmania.com ESMTP Exim 4.69 #1 Wed, 07 Dec 2011 13:32:08 -0800
220-We do not authorize the use of this system to transport unsolicited,
220 and/or bulk e-mail.