Hi ​@seadog - Thanks for reaching out!

Usually when we see these types of errors its mostly related to an issue on the Invitee’s end, either their email was entered incorrectly or there could be an issue with their server.

With Invitee notifications, you have two options:

1. With Calendar Invitations, Calendly will add an event to your calendar, then add the invitee as a guest. From there, your email will take care of sending the email to the user, so the notification comes from you directly. Depending on the user, this could get filtered as spam and rejected, its rare, but can happen. If you have an IT team, you can send them the undeliverable message you got to see if they can track down what happened.
2. Your other option is to use the Email Confirmation notification type, which lets Calendly send the notification directly to the user, instead of doing it through your Calendar. This also lets us see what went wrong in the event it fails to send to the customer. You can change this in your event settings here:

   

   

Give this a shot and let me know if it works! I’ll be happy to answer any questions you have.