How to design REST API for email sending service?

The scheme you have given is correct. Alternatively you can use controllers to perform some more complex actions.

In your case it can look like this:

(action)           (verb)   (URI)                             (type)
create:            POST   - /emails                         - collection
retrieve:          GET    - /email/{id}                     - resource
update:            PUT    - /email/{id}                     - resource
delete:            DELETE - /email/{id}                     - resource
send immediately:  POST   - /email/{id}/sendImmediately     - controller
just send:         POST   - /email/{id}/send                - controller
do something else: POST   - /email/{id}/someOtherActionType - controller

Note new controllers and the change creation works. The latter is rather subjective, but reasonable (as you cannot really access the URL of “no actual email” like I would interpret “/email” without “{id}” part).

Additional resources:

• REST API Design Rulebook