MicrosoftOnly thing that sounds familiar is that our environment was initially set to be internal-only. So, clients/guests could visit our Bookings page all day long and reserve a spot. But the emails wouldn't go through if they were external to our firm.
Once we figured this out, we worked with our internal I.T. folks to whitelist that on our side so that emails would be sent. I do not have the specifics or know what they changed, but our external Bookings get their emails now.
Not sure if this is your scenario, but maybe it'll help a little. Best of luck.
