Sorry, but I asked about a service that doesn't FIT the RESTful model.
Sending email is best mapped to POST (create), not PUT (update), and the URL doesn't change for POST.
What I find annoying is people objecting to "POST /actions/subscribe" (with the params in the post body) because it's "non-RESTful" -- the url is a verb and REST demands a noun.
That way you can list your subscriptions, modify, delete in a very consistent way, without a need for "POST /actions/modify_subscription" and such.
POST /actions/subscribe
and POST /subscriptions
and POST /abcdef1234567890
The shape of a URL has nothing to do with REST. However, if a lot of your URLs contain verbs it suggests that you aren't doing REST. It's a design odor.