[Dovecot] Logging subjects of messages

Dear Sirs, I want to know if there is any possibility to log the subjects of the messages downloaded from ther server. We use dovecot 1.0.15 with POP3S and IMAP and we want to keep a trace of the reading of messages by the users of our mail server (who, when and what are the subjects of the  e-mail messages read) because some users complain that they did not read some messages but in the logs of the server (maillog files( those messages appear to be delivered to their inboxes. Thank you very much for your help Best regards Alexandru Cucereanu