+ Design:
+
+ This should only work from a domain that exists in postfixadmin to a
+ domain that already exists in postfixadmin. If the target domain
+ does not exist, we should error out as soon as possible.
+
+ Once we're sure that the target domain does exist, we can hand off
+ the "mv" operation to the plugins. Some of them might have nothing
+ to do -- that's fine.
+
+ MAKE SURE WE DON'T OVERWRITE AN EXISTING USER!
+