A single attribute, "status" can be associated with the email messages or in gneral, the notifications.

It would constitute, therefore, the "state" of such messages or notifications.

Initially when they are "composed" but before being "submitted" for the "delivery", the attribute, "status" can assume the value, "readyforDelivery" with additional values. e.g., "readyToBeDelivered" subsequently.

POST /notifications would be used for "composing" the messages initially by setting the "status" to "readyforDelivery".

PATCH /notifications/{messageId} then, would provide the clients to initiate delivering the messages, which would change the "status" from "readyforDelivery" ---> "readyToBeDelivered"

This method does not require using verbs in the API Signatures, honors the REST principles for the HTTP Verbs, and free from any side- effects.