[exim] Sender address rejected: Domain not found

Hello all,

I am using exim as a mail hub for filtering and then passing to other
exchange servers.
I have recently turned off sender callouts as it was causing too much
confusion for some recipients :(

but now I have a new problem.. some of the recipient mail servers look
at the incoming email address and try to verify the domain, or mx (I'm
assuming.. )

I get things like this in the logs now:

2006-08-28 11:08:57 1GH7KP-0001Gq-Py == schoond@???
R=manual_route T=remote_smtp defer (-44): SMTP error from remote mail
server after RCPT TO:<schoond@???>: host 1.2.3.4[1.2.3.4]: 450
<tywo@???>: Sender address rejected: Domain not found

where 1.2.3.4 is a host that is ultimately accepting mail for
domain.org. So what I have been looking for, and keep finding sender
callouts as the answer, but I just want to verify that I could send them
mail.. (I guess is what the 1.2.3.4 box is doing

Doing a dnsmx on grounddesigns2000.com takes quite a long time..

Any help would be greatly appreciated.

Thanks.