I've found that if I put jibberish in there then delete it, the warning flag goes away. It will then let me close the show without a host email.
What's more annoying is that I have some contacts whose whole order history doesn't show under their name. The reason is that P3 says that the email listed under their name on a show is not in the proper format and it won't let me hit "Update Contact," But it won't let me fix it either because the show has been submitted.
Have you tried merging your contacts with like names? This will merge your history too..... just wondering