I, in UK, book things on Eventbrite, they email you with a calendar invite. Same with other booking systems for events IIRC. You can probably add people to an invitation? Maybe if you can exploit such a system then people would have them in their whitelist in any case?

A little adjacent to your question but relevant enough I think.

This is a regular part of the recruiting process, where you may start chatting in LinkedIn and then get an invite on your email.

I've received Apple Calendar invites containing Chinese characters from individuals I've never heard of. I deleted them, but just receiving them was a bit alarming.

Not unrealistic as a consultant. My boss sells me to a project. Then clients might be asked to send me the meeting invite to kick things of. I might not have directly communicated with client at any point at this time.

I recently booked a haircut that sent me a calendar invite via email after booking it. I had never interacted with that email before, but I accepted the invite.

Pretty often at work. I'm often interacting with client/vendor teams or even new people at the company I work for. Probably a few times a week I'll get an invite from someone I have never exchanged an actual email with. Maybe Teams/other chat messages, maybe exchanged information with one of their colleagues, or talked over the phone.

HR / Recruiter setting up interviews? The person doing the inviting might be different from previous calls/emails.

Customer meetings I get invited to often come from someone I’ve never dealt with before, but include others who I work with who were responsible for bringing me into it.

Project manager from other team arranging a cross team meeting?

Secretary office admin doing their job?