“Your message to firstname.lastname@example.org couldn’t be delivered”
Basically you’re going to have to talk to your email administrators to get it sorted. “Reply from customer/vendor received > From email address is “email@example.com”” SuiteAnswersId: 78277, “Undelivered Emails Saved Search” SuiteAnswers Id: 64668, and “Email Best Practices” SuiteAnswers Id: 9429 are good places to start if you need more detail for them.
The fact that you are receiving emails from firstname.lastname@example.org means one of the following:
- You don’t have DMARC/DKIM setup for your domain (which you should do, if you haven’t)
- The email is coming “from” a domain you do not control
Fixing the above may resolve the error or it may be something in your email setting or in the email settings of your recipient that is causing the issue. A quick googling of your error code indicates to me that it’s likely on the recipient side, as does the fact that this issue started only recently.