Cannot receive form notification emails

I’ve read several supports threads at WPFORMS. I cannot receive notification emails. The message below is the closest we got. Received this on a yahoo.com account. The administrator email in wordpress is the same yahoo.com account. We installed WP Mail SMTP plugin as suggested on your support forums and still no luck. php mail() is indeed working on our server. tested it with our own script and verified with host, it works. not sure what else to do :/

This message was created automatically by mail delivery software.

A message that you sent could not be delivered to one or more of its
recipients. This is a permanent error. The following address(es) failed:

[myemail]@gmail.com
host gmail-smtp-in.l.google.com [172.217.197.26]
SMTP error from remote mail server after end of data:
550-5.7.1 Unauthenticated email from yahoo.com is not accepted due to domain’s
550-5.7.1 DMARC policy. Please contact the administrator of yahoo.com domain if
550-5.7.1 this was a legitimate mail. Please visit
550-5.7.1 https://support.google.com/mail/answer/2451690 to learn about the
550 5.7.1 DMARC initiative. l1-v6si487997qvq.109 – gsmtp
This email was sent by Default (none) mailer, and generated by the WP Mail SMTP WordPress plugin.

The page I need help with: [log in to see the link]