On Sun, 02 January 2000, Owen DeLong wrote:

I'll take a stab at this:

Attempting to resolve a.b.c.d.e.f.g.h.i

If you get back a return value from DNS that says "Authoritative answer, a.b.c.d.e.f.g.h.i does not exist" Bounce the mail.

If you get back a return that says "Resolving a.b.c.d.e.f.g.h.i timed out without a return" Queue the mail.

This also works for i, h.i, g.h.i, f.g.h.i, etc.

I'm happy to agree this is a very sensible thing to do. However, there is no requirement for any mailer to do it. Some mailers may, but other mailers may not. So it is not very wise to assume all mailers work the same as yours. There is a written recommendation that domains should have DNS servers on seperate networks. There is no such recommendation that mailers retry or queue mail when they can't reach DNS servers.