I know this might be a dummy question or a question that comes from lack of knowledge, but I hope someone can still answer it. I did try to read a lot of Postfix documentation but found no answer to this. I don't even know if it's a Postfix specific or mail servers general question.
So I have a mail server, just a clean Postfix install that delivers email. I've defined my users and connected with IMAP and SMTP using Thunderbird.
When I went to Thunderbird account settings and disabled "place a copy", Postfix did not put a copy of the sent message in the user .Sent folder.
However, I've also connected my Gmail, Hotmail or Yahoo mail and disabled the "place a copy" and still have a copy in the sent items folder.
So in this case there are 2 options:
Postfix itself does not place copies of sent messages anywhere; it receives messages and delivers them to the recipient. Saving sent messages to your own mailbox is the responsibility of your user agent (Thunderbird, in your case).
It's important to understand that Postfix (and other traditional Unix SMTP servers) don't have a "user" concept. Yes, if so configured it's possible to authenticate by supplying a username and a password, but Postfix doesn't use this identity information.
That said, it's not impossible to configure Postfix to do what you expected – sender_bcc_maps can be used to add a recipient to messages sent by you, and by adding yourself and using a filter in your mail client (or mail delivery agent like procmail) you can make sure that messages sent by you end up in the Sent folder.