Sorry for being a little bit off topic in this comment:
It is remarks like these in comment #6 and comment #8 that are so immensly frustrating and demotivating!!
I fully understand that classic / legacy restart extensions have no future. But I don't understand why you have to kill restartless extensions as well - I can see no valid technical reason for this decision. Converting Mail Merge to a restartless extension would have been possible without too much work.
Nevermind... The way into the future is obviousy WebExtensions: But Thunderbird is apparently lacking a lot of APIs with very valid use-cases - like this specific API to send messages.
We add-on developers are told that we can use a technique named WebExtensions Experiments for the time being. But again Thunderbird is lacking a clear commitment that WebExtensions Experiments will be there for years to come!
For my own add-on Mail Merge this means that I can spend the next few weeks or months trying to solve or workaround all the problems I probably face by converting my add-on to a WebExtensions Experiment. And - assuming that I somehow manage this successfully - maybe one year later Thunderbird decides to kill WebExtensions Experiments without all the necessary WebExtensions APIs in place, then all my hard work was wasted.
Again, sorry for being a little bit off topic in this comment, the next two comments will be on topic - I promise!