example.com is good because it is explicitly reserved for this purpose and has no MX records.

Although very occasionally a service will check for MX records, but that is incredibly uncommon. My go-to email for public WiFi is [email protected] and have only been denied once (<1%)

I prefer webmaster@whatever site i'm on

A fair number of places will deny that, but I like to think it sends a message. I'm not sure how many, if any, domains still have a working webmaster@ address though.

That's a really good idea! I'll try it next time.