Hey Chris, couple of things going on in your email:
1. You're using the event ID field, and so the event ID is being rendered. You can use the event name field ({{contact.contact_msevtmgt_event_msevtmgt_originatingeventid.msevtmgt_name}} to render the name.
2. Please note that you're using the "Originating event" relationship. This only indicates the event that caused this contact to exist - this contact was created when this person registered for this event (Originating event). If you're looking to only have people attend events once, and to send this email out to ONLY those people whose contacts were created when they registered for the event (by filling out an event reg form) - then this approach will work.
However, if you're looking to create an actual "thank you for attending" email - you can't use the originating event relationship. What happens when someone attends Event 2 after attending event 1? Their originating event will still be Event 1.
