Here are changelog http://dovecot.org/list/dovecot-news...ay/000223.html
Code:
        * Session ID is now included by default in auth and login process
          log lines. It can be added to mail processes also by adding
          %{session} to mail_log_prefix.

        + Added ssl_require_crl setting, which specifies if CRL check must
          be successful when verifying client certificates.
        + Added mail_shared_explicit_inbox setting to specify if a shared INBOX
          should be accessible as "shared/$user" or "shared/$user/INBOX".
        - v2.1.5: Using "~/" as mail_location or elsewhere failed to actually
          expand it to home directory.
        - dbox: Fixed potential assert-crash when reading dbox files.
        - trash plugin: Fixed behavior when quota is already over limit.
        - mail_log plugin: Logging "copy" event didn't work.
        - Proxying to backend server with SSL: Verifying server certificate
          name always failed, because it was compared to an IP address.