It is possible to enable (force?) SMTP authentication using the 
$conf['mailer'] parameters in horde conf.php. In order to use the 
current logged in user's username and password, no username and 
password is specified in the conf file.
The problem is that the same settings are used for sending the email 
notifications for alerts, but the emails are sent from a cron script, 
so at that point no username and password are available, and the 
notifications fail with error message "authentication failed".

There should be a way to be able to use the above scenario.. (users 
send email using their own username and password and notification are 
sent either without authentication, or using a specific username and 
password?)