Yesterday, I used alpine to send a message to students. There were five
addresses on To, four of which were Gmail addresses.

I received an error message that Gmail rejected messages with more than
one address on To.

I got a reply from the student with the non-Gmail address.

I re-sent the message to just the students with Gmail addresses, put
them on Bcc, with the email address of another instructor on To. I have
no idea if that message was received but I didn't receive a rejection
message from Gmail.

Then I received a reply from one of the students with a Gmail address
but it was clearly in reply to the first message and not the second.
Despite the error message, the message had been delivered.

How does any user willingly use Gmail as a primary method of email
communication when the user has no idea what arbitrary rule Gmail will
apply preventing non-spam messages from being delivered?

I have a Gmail address but I use it to receive system messages and self
promotion from Google and as a backup communication method if all else
fails.