Jonathan, we're using CDOS Sys with excellent results.
You don't need Third-party software, neither Outlook or any client installed.
Please check this great article:

http://www.sweetpotatosoftware.com/SPSBlog/CommentView,guid,71acd54c-dcda-4dfa-b4ae-74854dd7947f.aspx


>Hi.
>I'm developing an application wich will run at night as a schedulled task in the server.. This application creates a log file with the results obtained. I would like to send an email with this file attached. Any suggestion?
>
>Thanks a lot in advance.