Google calendar invites sent from wisc.edu addresses to external addresses (i.e. anything NOT in the wisc.edu universe) is bouncing.
(For example: I sign in to google calendar as tnchristian@wisc.edu. I make a calendar event and invite LordsOfTheTrident@gmail.com. This invite email will bounce) 

This is due to a recent strengthening of Google’s inter-domain security. The people to fix this would be DoIT, but they’ve told us the fix won’t be coming anytime soon. Their current recommendation is to use Office365 (i.e. the calendar inside of outlook or outlook-on-the-web) to invite non-UW-email people to events.

Not a great workaround, as it involves two different calendars. If you’re routinely inviting non-wisc emails to events, for the time being you may wish to follow up with an email to these people and let them know.