In the good old days when one could telnet to SMTP servers, you could input several RCPT TO commands to define multiple recipients. I would guess all of the recipients would receive the exact same email.

This reference also shows https://www.samlogic.net/articles/smtp-commands-reference.ht... that you can define a "To:" header field, so even though the client shows "This email was sent to the following people" (by parsing the "To:" header), there's technically nothing stopping the mail client to lie in the header about who the mail was sent to. Or that 2 recipient could get 2 different versions of the e-mail.

I guess the mail client crafts a unique email (with a unique tracking pixel URL) and addresses it (via RCPT TO) to a unique recipient, but in the "To:" header it will specify all the recipients, claiming "Each of the following people got an exact copy of the e-mail".