I had an inexplicable problem setting up the mail server for a Plone 3 site recently. The site was at an institution with port 25 disabled and it required, as far as I could tell, the plone mail server to authenticate with the institutional SMTP server. My first and unsuccessful attempt was to navigate to Site Setup -> Mail Settings -> Mail server and set the SMTP server to the institutional server, the SMTP port to 25, and enter the "appropriate" authentication information as if it were for ESMTP. This did not work! Upon trying to send mail through plone (in this case by setting up an account) I received an error from the SMTP server that it required authentication and that email clients should specify port 465. Simply setting the SMTP port 465 in the plone "Mail settings" page did not work either. After a little bit of struggle, I found again that the ZMI proved invaluable. Here's what I did:

1. Log in as the plone admin
2. Click "Site Setup" in the upper right hand corner
3. Click "Zope Management Interface" under the "Plone Configuration" heading
4. Click the "MailHost" item
5. Enter the following values
1. Title - fully.qualified.smtp.mail.host.name.com (e.g. "smtp.somecompany.com")
2. SMTP Host - fully.qualified.smtp.mail.host.name.com (e.g. "smtp.somecompany.com")
3. SMTP Port - 25
   
4. SMTP Userid - myaccount@somecompany.com (the "@somecompany.com" might not be required for some SMTP servers)
5. SMTP Password - ********* (<== that's a real password, but you just can't see it)
6. Disable TLS - Unchecked

Strangely enough, setting the authentication information specified through the ZMI is different from the information specified through the plone "Mail Settings" page. Also strange was that port 25 was required instead of port 465 like the institution claimed to require. I'm not sure of the source of this latter discrepancy.