AuthSMTP is an authenticated SMTP relay service that you can use with web applications or any situation you need to send outbound e-mail. Because it is an authenticated service it is a little trickier to configure Postfix to relay through their service. I found
this post really helpful in configuring the sasl options but one thing I couldn't find a clear answer on was how to use a port other than 25 for the relay host. AuthSMTP offers alternative ports (23, 26, 2525) for SMTP because some ISP's block port 25. To use an alternative port just put a colon after the host name and add the port number. Like this (in main.cf):
relayhost = mail.authsmtp.com:2525
The entry in your sasl-passwords file must match the relayhost name like this:
mail.authsmtp.com:2525 username:secretpassword
Just a quick tip about something that wasn't obvious to me and hopefully this helps out someone else.
