Similar to gmail's API ability to create new email messages inside a mailbox via the API, it would be ideal to have API control to insert and import messages into a mailbox. The API gmail has our what follows:
https://developers.google.com/gmail/api/v1/reference/users/messages/import
https://developers.google.com/gmail/api/v1/reference/users/messages/insert

These allow for ability to create new messages inside a mailbox without having to send emails to a user.