Good call Jason ...
I've got a dedicated server and have had the server locked down to only accept connections from my 3rd party spam filtering company, in order to prevent spam attacks and brut force mail out attacks.
I need to ask the authsmtp people what the sending ip address is and then I can add it to my firewall allowed list (currently set to only spam filtering company).
For info, I was sending the newsletter via normal php mail functions but spam filtering company requested I use a bulk mail server to help protect their ip address white listing, hence the conversion to PEAR mail
Hopefully will be able to report back with a happy outcome soon !