Exchange server – Undelivered emails – 421 4.4.2 Connection dropped

I have done a setup just like you are asking, albeit with Exchange 2010 which is somewhat different than 2007 in this regard. It will handle distro lists, etc. Be cautious about having the same ones cross-forest though...like helpdesk or sales or whatever.

Basically it comes down to 3 things:

1. You should (if you don't already) have a shared SMTP namespace for DEF.NET. This will make sending and receiving as DEF.NET for EVERYONE...and simplify the namespaces used. You can do this cross-forest.
2. Exchange 2010 simplifies this part, but it is still doable in Exchange 2007 with the latest SP I believe. Look into Cross-Forest Federation, if needed. Basically this will allow for cross-forest resource availability and free/busy sync. Helps for meetings, etc
3. We used (and I highly recommend) a product by netsec.de called GAL Sync...http://www.netsec.de/en/produkte/galsync/ It is made by a German company but it has full english support and works amazingly well. It will require setting it up on both sides, but it works so well with so many customizable options. We had looked into all the others, like IIFP (now ILM), Quest Collaboration Services, doing it in house, manual contacts on both sides, etc.

This isn't for the faint of heart overall though. You have to really make sure you do it right, because Exchange is finicky. Take for instance a single contact you already have on the other forest. Now delete it and recreate it and watch what happens when someone tries to reply to an existing email from the original "contact". It will bounce because it is looking for that original Exchange object, not the smtp address. So be careful and go slow. Even with the above options, it took me a few months of planning and testing to make sure go live would go smoothly.