Outlook has been doing this since 2008 that I know about. I wrote a mass-email program in my FileMaker database. I wrote and tested on MacOS. Worked great. No prompts, just sent the 10 samples out. Running on Windows7 with Outlook, it prompted for each one. The sender had to click OK for all 499 emails. Still faster than doing it manually. We use Filemaker to send our AR invoices, we are prompted by Outlook 2019 for each one.
The explanation is that MS is protecting US from some third-party program taking active control and spamming. There are mail services that you can use to send mass emails. I do not remember who, but there was a discussion on the FIleMaker Message Boards. I remember one allowed up to 25,000 emails per month free.
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