> Differentiating between cases 1 and 2 would require a list of domains > known to the MTA, which of course I could keep in a file or a database, > but Postfix already has this information, and I would like to avoid > "duplicating" it.
Why not query the same database?
Postfix's database proxy service is not designed for use by non-Postfix
programs. Programs that use this are unsupported, meaning they will
whenever this internal protocol is changed.
Wietse
