Tracking mail delivery to a user, perhaps?

Question

Mail server

Colleagues, hello.
There is a postfix + dovecot mail server.
It is necessary to be able to determine from the logs whether a particular letter was picked up by the user's mail client.
For the case when the user says that he did not receive the letter. According to the logs, I see that the letter definitely reached the mail server and went to the INBOX folder of the addressee.
Picking the dovecot configs did not give a positive result, a lot of information is written to the log, but in fact nothing useful.
We work on POP3.
do not offer IMAP)

Answer 1

There is a RETR n command in the POP3 protocol, where n is the identifier of the incoming message. This is the maximum that can be offered, see the mail server logs.

Answer 2

Not exactly according to the logs, but there is a way.
When an email is delivered to a mailbox and not read by an email client, it is located in
/home//Maildir/new (if the format for storing emails is maildir) . If the message is read, it is moved to another folder or deleted. If it is known from the logs that the message was delivered to the mailbox, but it is not in /Maildir/new, then the mail client "picked it up". However, do not forget that the mail client could independently mark it as spam and move it to the appropriate folder already in the mail client.
If the mail storage format is mbox, then new letters are written to the end of the mail file. I cannot say how a new message is allocated in the mbox file, and whether it is allocated at all. However, if the POP3 client deletes messages on the server after receiving, then the sign of receiving a letter is the absence of this letter in the mbox file.