I'm one of those who moved from Gmail to Fastmail and the only thing I really miss is bulk sending, click tracking and other features GMass used to provide.

So me and my team figured out Fastmails API (how the frontend works plus JMAP) and implemented the "Mass Send" button straight into the Fastmail UI. We've got many features still to come like auto-followup, mail merge open/click-tracking.

So if you're using Fastmail give it a go!

Chrome: https://chrome.google.com/webstore/detail/massfastmail/gbdfagckfacojiaicjhngpgepjofchhi

Firefox: https://addons.mozilla.org/en-US/firefox/addon/massfastmail/