Suppose your ISP relay host is "mailrelay.example.com" and the user
name and password to use there is "user" and "password" respectively.
Enable SMTP authentication on outbound messages:
$ postconf -e smtp_sasl_auth_enable=yes
Create a file text file mapping which name/password should be used for
which given outbound destination. This file can contain multiple
$ echo mailrelay.example.com user
assword > /opt/zimbra/conf/relay_password
Create a postfix lookup table from this text file:
$ postmap /opt/zimbra/conf/relay_password
Test that the map is okay and that the user: password pair is returned.
$ postmap -q mailrelay.example.com /opt/zimbra/conf/relay_password
Make postfix use this:
$ postconf -e smtp_sasl_password_maps=hash:/opt/zimbra/conf/relay_password
$ postfix reload