Is your port 25 blocked ?
Someday your beloved port 25 will be blocked by your ISP, at least for outgoing trafic. If you're a Postfix user, let's see how to circumvent this annoying limitation...
1) You have a local Postfix server that wants to talk to the outside world
You have to use your ISP's SMTP server as a relay, with or without authentification.
Add a 'relayhost' directive to /etc/postfix/main.cf
For my ISP (orange.fr)
relayhost = [smtp-msa.orange.fr]:587
If the relayhost needs authentification, add these directives to main.cf
smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/sasl_client
smtp_sasl_security_options = noanonymous
create a new file /etc/postfix/sasl_client
edit this file and add an entry like
[smtp-msa.orange.fr]:587 mylogin:mypass
mylogin:mypass => SMTP login and pass
run postmap sasl_client (will create sasl_client.db file)
service postfix reload
2) Your MUA wants to reach your outside Postfix server ?
On your server, edit /etc/postfix/master.cf and uncomment the line starting with 'submission inet', like this :
submission inet n - n - - smtpd
-o smtpd_etrn_restrictions=reject
-o smtpd_client_restrictions=permit_sasl_authenticated,reject
service postfix reload
Configure your MUA (eg. thunderbird) to use the port 587