Under the MM Forms Mail Options tab you must have the from field match the email set with WP-Mail-SMTP. At least in my case you do.
I was trying to set that to the user-entered information. Not sure why this method would only break with Yahoo and Gmail addresses, but my solution will work. I’d much rather the recipient of the form be able to just hit reply to respond to the user, but I can just have them reply to the email included in the body for now.
If anyone can explain why the other method is breaking, I’m all ears.