Cross Forest Mail Flow

Question

I have a situation where we have 2 companies merging together.  We have users in forest 1 and users in forest 2.  Is it possible to have a user in Forest 1 who can send emails with an email domain from Forest 2? 
For sending an email i would create a send connector to route emails over to Forest2 email servers and have them send to the outside world.  But receiving emails is where i'm stuck.  The MX record would point the email into Forest2 but we would
need the email to be routed to the users mailbox in forest1.  
Is this possible? 

Answer 1

If any email system is authoritative for an SMTP namespace, it is responsible for sending non-delivery reports (NDRs) for any message sent to an address in that SMTP namespace that it doesn't have in its directory.  And you can't share an SMTP
namespace between two email systems without having at least one of them as authoritative for the namespace - one of them needs to own the namespace so that if a message has an invalid recipient, the email will get an NDR.
In the steps I outlined above, you have one Exchange system (Forest1) that has a namespace it is not authoritative for (the Forest2 namespace) already. You can't also set Forest2 as non-authoritative for the namespace, or you risk mail loops.  So for
each mailbox in Forest1 that you need to set a Forest2 SMTP address on, you need to have a way for Forest2 to send email to that mailbox.  This is why you need contacts.  Delete the contacts, and you can no longer send from Forest2 to the mailboxes
with Forest2 addresses that reside in Forest1 - Forest2 has no recipients with that SMTP address, so it will bound the message (note - this includes inbound messages from the Internet, since all messages bound for the Forest2 email address are first delivered
to the Forest2 email system).
In the example you mention, you are sharing the namespace with a third-party email system.  In that example, that third-party email system is authoritative for the namespace and is responsible for the NDR.