I had an issue with my provider where any mail leaving the server requires a specific SMTP server so mails generated from zenphoto goes to limbo. I finally fixed that by modifying the zp_mail function in functions.php with PHPMailer.

So for those of you that want to select a specific SMTP, S/MIME or authentication, I'd suggest looking into incorporating PHPMailer with your installation!

http://sourceforge.net/projects/phpmailer
http://phpmailer.codeworxtech.com/

Oh I'm using an old customized version of zp (1.1.5) so please correct me if the latest zenphoto already has this built in!

We have not changed zenphoto. If you get around to updating this implementation to the current release please submit a ticket with your changes. (At this point you might as well wait a few weeks, though. August 15 is the roadmap date for the 1.2 release.)