What is DMARC

https://en.wikipedia.org/wiki/DMARC

How am I affected?

If you utilize a form on your website which has visitors send an email using a 'FROM' field in which they input their email addresses, there is a chance that the email will fail due to authentication if DMARC is being used by the @domain.com address. As of this writing, this only affects yahoo.com addresses, but this may change in the future.

Note: Emails which are rejected due to the DMARC policy will normally indicate as much within the error itself. An example of an email rejected by Gmail is like below:

exim_mainlog-20160731.gz:2016-07-25 16:52:32 1bRZkV-002dAa-1j ** abc@gmail.com (booking@abc.com.au) <booking@abc.com.au> R=lookuphost T=remote_smtp H=gmail-smtp-in.l.google.com [64.233.189.26] X=TLSv1.2:ECDHE-RSA-AES128-GCM-SHA256:128 CV=yes: SMTP error from remote mail server after end of data: 550-5.7.1 Unauthenticated email from yahoo.com.au is not accepted due to\n550-5.7.1 domain's DMARC policy. Please contact administrator of yahoo.com.au\n550-5.7.1 domain if this was a legitimate mail. Please visit\n550-5.7.1  https://support.google.com/mail/answer/2451690 to learn about DMARC\n550 5.7.1 initiative. zi9si31993244pac.192 - gsmtp

What can I do to fix this?

Our suggesting is that have your programmer update the 'FROM' field within your form to use an email address of @yourdomain.com. Additionally, the 'REPLY-TO' setting can be used with any DMARC compliant address and will successfully send emails without rejection such as @yahoo.com, @hotmail.com, @gmail.com, or @aol.com.
An example can be seen in the following header:

$headers = 'From: user@yourdomain.com' . " " .
'Reply-To: user@yahoo.com' . " " .
'X-Mailer: PHP/' . phpversion();