alternative smtp server from smartlist

Normally I use sendmail on my own server for sending smartlist mail. However, for special reasons, I now need to use a remote smtp server for one particular list. I could not get the sendmail documentation to reveal to me the secret for doing that. And presuming that I can find it, then what would be sensible way to inform smartlist about it. I have guessed someting along the lines of using rc.custom and setting $sendmailOPT. Thanks for your advice. Sincerely, Seth Chaiklin