I do believe that ALL upfront sites will bounce all email that come from yahoo emails if using gmail.
Reviewing the mail server error messages:
Oct 25 22:48:08 myserver postfix/smtp: CE92F6085D: to=<email@example.com>, relay=gmail-smtp-in.l.google.com[74.XXX.28.27]:25, delay=0.51, delays=0.01/0/0.34/0.16, dsn=5.7.1, status=bounced (host gmail-smtp-in.l.google.com[74.XXX.28.27] said: 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. g9si20073618pgn.302 - gsmtp (in reply to end of DATA command))
So...a possible solution to the developer to implement TODAY:
Review the method the word press plugin "contact form 7" uses.
Have the sender's email be used as a reply-to email (not the from). The from email should be from something originating from the server. Perhaps customized by the network admin or the site admin.
The scary thing is all your users have no idea this is happening, so I do hope for transparency in responsibly communicating the danger of this issue to your users asap.