I would not go with a specific client unless you have plenty of control that this client will never change (something you should not want to tie to).
Take a look at this link
] in a website from one of the members of CP (the article here is not as new as in his web site).
Here you will find some classes (look into the code, there are all the dependencies well explained) that will help you to send e-mails without even needing to install outlook or any other client in your machine.
The best of it is that the safe measures established by Outlook (asking for each message that an external app wants to send) are automatically gone, making this perfect for unattended sending e-mails.
Hope this helps you.