The keys used are:
!: modified feature, -: deleted feature, +: new feature
-1.5.17 (2007-11-01)
+1.5.17 (2007-11-01):
! --enable-exact-address works again
+
+1.5.16 (2007-06-09):
+
+ next-unread-mailbox
+ $message_cache_clean (clean cache on sync)
- + %P expando for $pager_format
+ + $smtp_pass
+ ! $header_cache_compress defaults to yes
-1.5.15 (2007-04-06)
+1.5.15 (2007-04-06):
- $imap_home_namespace (useless clutter)
+ $check_mbox_size (use size change instead of atime for new mail)