Question about local delivery

I'm running the following:

Sun Java(tm) System Messaging Server 6.2-2.05 (built Apr 28 2005) 
libimta.so 6.2-2.05 (built 20:29:38, Apr 28 2005)

Dirserv 5.2, patch 3.

Short form:

I'm trying to see if it's possible to change the way the MTA handled 
delivering local messages for a particular domain. Is there a way to 
configure the MTA to first check if an address exists in its local ldap 
directory and if so, handle it locally, without making the server 
"authoritative" for a particular domain? In other words: if user@domain 
exists on the server, handle it locally; otherwise, send it outbound for 
delivery.

Long form (In case there's a better way to do it):

We have two domains, A and B, whose MX points to our virus 
scanning/spam blocking/outside MTA. Once accepted by that MTA, mail for 
domain A gets routed to the JES server, while domain B gets routed to 
the customer's own MTA. In the past, our outside MTA has done address 
translation, rewriting some addresses in domain B (the customer's MTA) 
to domain A (the one handled by our MTA.) Unfortunately, that outside 
MTA is being retired and is being replaced by a new outside MTA 
incapable of address translation. As a plus, the new MTA is capable of 
doing address-based routing, so specific addresses can be routed to 
particular hosts.

So, in an ideal situation, the new MTA would route the domain B 
addresses that used to be rewritten over to the JES server, which could 
accept them since the users could have mailalternateaddresses of domain 
A. In order to make this work, though, the JES MTA must send on any mail 
for domain B not in its local directory, so that they can be properly 
routed over to the customer's own MTA.

In practice, the MTA will not attempt a local delivery, but will simply 
route outbound all mail for domain B, regardless of local directory 
addresses. This appears, from my read of the admin guide, to be working 
correctly. The problem is that if the JES server becomes authoritative 
for domain B, any mail sent via webmail or from expanding groups for 
non-local domain A addresses will receive a NDR and not reach their 
destination. If the server isn't authoritative for domain B, it simply 
routes all the mail for domain B back out to the internet.

Simply rewriting all outbound mail from domain B to domain A won't work, 
since there's mail for domain B coming out from the messaging server. I 
can think of some ways to short circuit the NDR process, but I want it 
to only apply to one domain (domain B.)

So, is there a way to handle this situation using the MTA?

Dan Taylor