Multiple possibilities
- Antivirus software is blocking traffic going to port 25
- Gateway/firewall is blocking
- ISP is blocking
All the above are common causes.
If you are trying to use that server as your out going smtp, use port 587 or 465.
You may have to modify postfix configuration file your email server to enable the above ports.
In /etc/postfix/master.cf, look for following lines
#submission inet n - - - - smtpd
# -o smtpd_tls_security_level=encrypt
# -o smtpd_sasl_auth_enable=yes
# -o smtpd_client_restrictions=permit_sasl_authenticated,reject
# -o milter_macro_daemon_name=ORIGINATING
#smtps inet n - - - - smtpd
# -o smtpd_tls_wrappermode=yes
# -o smtpd_sasl_auth_enable=yes
# -o smtpd_client_restrictions=permit_sasl_authenticated,reject
# -o milter_macro_daemon_name=ORIGINATING
Un-comment them (remove the # in the beginning of each line) and restart postfix.
The submission section enable port 587.
the smtps section enable port 465.
You sir, are a legend. – Nick Duffell – 2012-11-08T08:54:18.877