Cross-Forest Migration NDR Message

Working towards a cross-forest migration from sub.source.com and dest.com.  Both domains have the same primary e-mail address domain currently, both are on Exchange 2010 (sub.source.com no updates, dest.com SP3).  The migration is being done in 3 sections by location over a 2-3 week period, ~50 users, ~80 users, ~200 users.

The process we're following via custom powershell script is prepare-mailbox script, ADMT users over including passwords though excluding exchange attributes, and then move mailbox from sub.source.com to dest.com.  After the move

- Can login with previous password
- All data is moved
- New messages both ways work
- Old e-mail replies from sub.source.com work to dest.com
- Old e-mail replies from dest.com to sub.source.com don't work.

The X500 address moves over properly along with other necessary Exchange attributes to dest.com.  I believe due to the way Exchange internally routes e-mail it's get's hung up replying to the internal address of the sub.source.com users as both sides are the same e-mail domain name.  It sounds like it's looking at the hub transport, tries to find the old user based on internal address, errors out as that users doesn't exist in dest.com, and NDRs.

Any tips on how to get past this?  Temporary contacts or perhaps there's something I'm missing?

Thank you for your help and this is pre-migration so fortunately testing has shown the issue beforehand.  I am aware the GAL won't be the same and free/busy will have some problems.  Anything else that you guys can foresee as issues?