Two ways I can think of offhand
1. create a form on your web server an submit the email through that with an http post.
2. use the new sockets functions in brightscript to make a direct smtp connection to your server (must be on a port other than 25 most isp's block that port.)
For the former, there are plenty of scripts on the internet, just use google, and you would use AsyncPostFromString method of roURLTransfer.
For the latter, you would have to spend some time understanding the smtp protocol and implmenting it on roku, probably not that easy to do, but should be possible.
- Joel