Rated 3 out of 5 stars

My opinion is based on the discovered error: does not save sent copy in Sent, nor in Sent in local folders, also, no chance to set the exact sending time.
Eventually, I checked the native TB option through Menu > Send unsent messages, and it works smoothly, saving a copy in the Sent folder of the acount from which the email was sent.

This review is for a previous version of the add-on (6.0.4). 

The button provides NO own send functions. It only calls the native send now function. If the mails are not saved, then the native function has an issue.