how do I change postfix port from 25 to 587?

Solution 1:

According to your comments on other answers, you need to configure Postfix to use Gmail as a relay host. There are many tutorials on the Internet for this; here's a quick version.

Note: With this configuration, all mail must be sent using your Gmail address as "From".

1. Undo all your changes to master.cf.

2. In main.cf, add these settings:

   # This tells Postfix to hand off all messages to Gmail, and never do direct delivery.
   relayhost = [smtp.gmail.com]:587
   
   # This enables TLS (SMTPS) certificate verification, because Gmail has a valid one.
   smtp_tls_security_level = verify
   smtp_tls_CAfile = /etc/ssl/certs/ca-certificates.crt
   smtp_tls_session_cache_database = btree:/var/run/smtp_tls_session_cache
   
   # This tells Postfix to provide the username/password when Gmail asks for one.
   smtp_sasl_auth_enable = yes
   smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
   smtp_sasl_security_options = noanonymous
   

3. In /etc/postfix/sasl_passwd, add your Gmail username and password, like this:

   [smtp.gmail.com]:587    [email protected]:mypassword
   

4. Compile the sasl_passwd file into a database:

   postmap /etc/postfix/sasl_passwd
   

5. Finally reload Postfix's main configuration:

   postfix reload
   

Solution 2:

If you only want it running on port 587 (and I'm not sure you do; I'd think you'd want it running on both 25 and 587), then find the line in /etc/postfix/master.cf that looks like this:

smtp      inet  n       -       n       -       -       smtpd

And change it to look like this:

587      inet  n       -       n       -       -       smtpd

If you want it running on both ports, then add the second line after the first one rather than replacing it.