newsletters not received by some

I have learned that (at least) three members stopped receiving MailPoet newsletters in late October (gmail, yahoo and aol). There are likely more, but I can’t identify them, right? (“Let me know if you don’t receive this message.”) I assume most of the members do receive the newsletters (I do, and so do the several others I have contacted). And up until late October, the three also rec’d all newsletters. All three people are confirmed and on the newsletter mail list. No bounces.

I do not know if the newsletters are not being sent or just not being received.

I went to the HELP tab. I had all the default settings. But upon reading “What is the Newsletter Task Schedule?” I went to Settings>Advanced>Newsletter task schedule cron and changed from “Visitors to your website (recommended)” to “Mail Poet’s own script.” Did not solve the problem so changed back to recommended setting. (I have absolutely no idea what either choice means, but I tried…)

Settings: Send message with: Mail Poet Sending Service
MailPoet 3.74.1. Windows. Hostgator.

I am a volunteer webmaster. Assume I know nothing.

What additional info do you need?
Thank you,
WAJCGS