/etc/apparmor.d/abstractions/postfix-common is in apparmor 2.10.95-0ubuntu2.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
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#
#    Copyright (C) 2002-2005 Novell/SUSE
#    Copyright (C) 2015 Canonical, Ltd.
#
#    This program is free software; you can redistribute it and/or
#    modify it under the terms of version 2 of the GNU General Public
#    License published by the Free Software Foundation.
#
# ------------------------------------------------------------------
# used with postfix/*
  capability            setuid,
  capability            setgid,
  capability            sys_chroot,
  # postfix's master can send us signals
  signal receive peer=/usr/lib/postfix/master,
  unix (send, receive) peer=(label=/usr/lib/postfix/master),
  /etc/mailname         r,
  /etc/postfix/*.cf     r,
  /etc/postfix/*.db     r,
  @{PROC}/net/if_inet6  r,
  /usr/lib/postfix/*.so mr,
  /usr/lib{,32,64}/sasl2/*    mr,
  /usr/lib{,32,64}/sasl2/     r,
  /usr/lib/@{multiarch}/sasl2/*      mr,
  /usr/lib/@{multiarch}/sasl2/       r,
  /var/spool/postfix/etc/*        r,
  /var/spool/postfix/lib/lib*.so* mr,
  /var/spool/postfix/lib/@{multiarch}/lib*.so* mr,
 |