You can configure Postfix to deliver mail by relaying it through your ISPs mail server instead trying to handle it yourself. This can be helpful to work around IP blocks, reverse DNS limitations, spam blocks, etc. Simply add a relayhost directive to your /etc/postfix/main.cf config file like this:

relayhost = mail.domain.com

You can also setup SMTP authentication if you SMTP server requires it.

I had a server that Postfix wasn't running on, but had 8500 old emails in the queue. I needed to remove them since they weren't relevant anymore.

postsuper -d ALL