Seeing an error for more people lately when sending to a Google account. On the one hand it looks like the error is saying you are not authenticated, but then it seems to talk about a DMARC record missing. For this error I did have a DMARC record and MXToolbox gave it a green check, I changed to rejecting 20%.

Remote Server returned: '550 5.7.26 Unauthenticated email from domain1.com is not accepted due to 5.7.26 domain's DMARC policy. Please contact the administrator of 5.7.26 domain1.com domain if this was a legitimate mail. Please visit 5.7.26 https://support.google.com/mail/answer/2451690 to learn about the 5.7.26 DMARC initiative. y23si13182197jai.77 - gsmtp'

The link did not provide a lot of good info.

Anyone with thoughts or ideas? I will see if there are any issues today after changing their DMARC record late yesterday afternoon.