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.