When a message fails DMARC authentication checks at a receiver (such as Yahoo, Google, Comcast, etc), and the domain is configured for DMARC-reject, the message is usually rejected during the SMTP conversation (before it is fully accepted). To the sender of the message, the reason that the message was not accepted can look different depending on who the target receiver was. If the target recipient is hosted by Google, the rejection will look like:
<email@example.com>: host ASPMX.L.GOOGLE.com[18.104.22.168] said: 550-5.7.1 Unauthenticated email from tomki.com is not accepted due to domain's 550-5.7.1 DMARC policy. Please contact administrator of tomki.com domain if 550-5.7.1 this was a legitimate mail. Please visit 550-5.7.1 http://support.google.com/mail/answer/2451690 to learn about DMARC 550 5.7.1 initiative. hs9si12099412pbc.34 - gsmtp (in reply to end of DATA command)
If the target recipient is hosted by Yahoo, the rejection may look like:
554 5.7.9 Message not accepted for policy reasons. See http://postmaster.yahoo.com/errors/postmaster-28.html
Other receivers may respond with messages either more or less informative.
Because these messages are rejected before being received by the destination, the responsibility for generating a bounce message back to the original sender lies with the e-mail system attempting to send the message.
So it is important to understand that the receiver (Yahoo/Google/etc) did not receive the message, and did not drop the message - it was rejected.