Add a post_upgrade notification about the change to lmdb.

postfix.install

@@ -5,4 +5,9 @@ post_upgrade() {
   if [ "$(vercmp "$2" "3.5.9")" -le 0 ]; then
     echo "WARNING: The following features are now only available via optdepends: cdb, ldap, lmdb, mysql, pcre, pgsql, sqlite."
   fi
+  # switch to lmdb
+  if [ "$(vercmp "$2" "3.9.0")" -le 0 ]; then
+    echo "WARNING: Support for BDB hash: and btree: database types has been removed, the default database type is now lmdb."
+    echo "Please update your main.cf configuration accordingly, and use postmap to convert existing databases."
+  fi
 }