The mbox from-line, not to be confused with the From: header, is simply not part of the email message but used only as a separator. Heck, it need not even contain valid information, but only proper formatting to satisfy picky/"smart" MUA's. Usually, it conveniently does carry some useful information, but e.g. when the SMTP envelope-from is '<>' then the mbox from-line usually contains something like MAILER-DAEMON to stay within the formatting specification.
Once again: I've never asked for an explanation about what that From-Line is.
I did not ask whether it contains useful information either.
I've asked whether dovecot allows to retrieve it over IMAP.
Why is it impossible for you to understand the question before answering?
Most people do not want an answer to the question they asked, but want a solution to their problem.
I did not ask for a solution of a problem, either.
I've asked for a precise answer exactly to the question I've asked.
That's why you fail to focus on the question and to answer it. Because you want to sell your solution for what you believe the problem is - or what you like it to be. But your assumption about the problem is completely wrong. So is your answer.
Stop obtruding solutions that nobody has asked for.