On a survey with 5000+ registered participants, we've had a few public registrations where the participant sees the message:

Thank you for registering to participate in this survey.
You are registered but an error happened when trying to send the email....

We verified that the email address is valid, and all other emails around the same time were sent.

Can someone shed light on what specifically would trigger this error message?

I see this is found in the PHP page:

github.com/LimeSurvey/LimeSurvey/blob/ma...gisterController.php