Questions Acumatica transactional emails going to customer spam boxes

I'm not an email expert, but this sounds like an "email problem" rather than an "Acumatica problem" since it sounds like Acumatica is sending the email properly.

There is a lot of mystery that I don't understand regarding what happens between email leaving Acumatica and arriving in someone's inbox (or SPAM folder).

It depends on the server that sends the email, how you authenticated with the server, the domain that it's sent to and whether the domain thinks it's SPAM, the email client that it's sent to and whether the email client thinks it's SPAM, and whether you stood on one foot jumping up and down while holding a lucky rabbit's foot when the email was sent (ok, made that last part up).

Here's an article that maybe shines a little light on the mystery:

https://sendgrid.com/blog/why-are-my-transactional-emails-ending-up-in-the-spam-folder/

Interesting!!  Thank you so much, I appreciate the info and the article.