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