Need help configuring SendMail network ports from Webmin/Virtualmin!

I can receive email just fine, and PHP can send mail directly through SendMail, but for some reason I can't send email over port 25 or 465 (using SSL).

Preferably, I'd like to be able to send email over port 465 using SSL. From "Network Ports", I added port 465 on all addresses. My network ports table now looks like this:

Port name ....... Listen on address ...... Listen on port
-----------------------------------------------
SMTP ...................... All ....................... Default (25)
SMTPS ..................... All ........................... 465

(port options are all unchecked)

I've tried sending email on port 25 without SSL and on port 465 with SSL, but nothing works.

On port 25, I immediately get an error saying "Sending of message failed. An error occurred sending mail: Unable to authenticate to SMTP server smtp.prudlow.com. The server does not support any compatible insecure authentication mechanism but you have chosen insecure authentication. Try switching on secure authentication or contact your service provider."

On port 465, it just keeps "sending..." and "sending..." and "sending..." forever.

Also, in Thunderbird, it never asks me for a password (the username is set beforehand; usually it will prompt for a password when you first send a message on a new SMTP).

What am I doing wrong???

Thanks,
Grant