I need to get reason statistics for all bounced emails from mail.log.
Which reason types exist?
A "Bounce" message is just a normal email message, with the distinction that it was forwarded to the originating address in response to a failure condition somewhere in the delivery path; furthermore, the intervening mail hosts were unable to rectify this failure condition, hence the message is returned to the sender as undeliverable.
A bounce is neither a status, nor a failure reason.
Your own mail server log will only contain a possible reason if your mail server was responsible for generating the bounce message.