I have been testing SendSimple app for a little over a week now after having had some initial issues getting things setup correctly. Turned out that you must be logged in with the exact chrome profile as your email. The founder was very helpfull and inmediately proposed a quick zoom meeting to explain how to configure the tool. I have done some further testing with both (free) gmail accounts as well as my own smtp settings and everything is working fine. What I like about sendsimple app is it's simplicity. Just a google sheet with a list of emails and your good to go!