Gmail has recently decided to embrace DMARC more explicitly, much like their email brethren, Yahoo and AOL. You may have started seeing bounce messages with the Reason of:

550 5.7.1 Unauthenticated email from domain.tld is not accepted due to domain’s DMARC policy. Please contact administrator of domain.tld domain if this was a legitimate mail. Please visit https://support.google.com/mail/answer/2451690 to learn about DMARC initiative. 62si14044909itw.103 - gsmtp

This is because Gmail will no longer be accepting messages where the From domain is a Gmail address and the message originates from a non-approved Gmail mail domain server/service. This is a security measure they have implemented to help reduce potential address spoofing of their mail domains.

What this means: As of June, 2016, you can no longer send with the From address being anything from a Gmail address when sending to a domain that checks DMARC before accepting mail.

What you can do: You will need to change the From address you use in emails to a non-Gmail address. We recommend using one at your own mail domain, or one you control that is legitimate. You can then set the Reply-To field to be the original Gmail address that previously was used in the From field.

What about these messages, are they lost? : Yes, any send with this bounce message is discarded and tracked as a Block. You will need to adjust your From address field settings, and then try resending from your side.

For more extended reading, check out this article on the DMARC changes, as well as our DMARC Classroom article.