More and more internet access providers are closing port 25 to reduce spam except for connections to their own mail servers. If you run your own email server and have problems connecting to it on port 25, you can enable port 465 (SMTPS) in postfix as a workaround.
Edit the /etc/postfix/master.cf file:
nano /etc/postfix/master.cf
and remove the # in front of the smtps line. Example for Debain 5, change the line:
#smtps inet n - - - - smtpd
to:
smtps inet n - - - - smtpd
and restart postfix:
service postfix restart