Letters without headers to, cc, bcc. How do they do it?

Question

Email

It seems my knowledge of e-mail is outdated.
I receive letters where I am not in the recipients, and there are no to, cc, bcc headers at all.
When viewing the original letter (in gmail), I see my address only in the Delivered-To header.
Letters are sent by various acquaintances through mail.ru. Apparently there is some kind of function for mailings, this is indicated by the text of the letter.
But how does it work? How does the letter reach the addressee if it is not specified?

Answer 1

Bcc is hidden on the sender side. In the SMTP protocol, the RCPT TO command is not used to specify the recipient of the message, but the RCPT TO command: