Now when Microsoft has released guidance for multi-tenancy on Exchange 2010 SP2 using Address Book Policies I started to explore the possibilites of implementing it.
There is one part of the guidance document that relates to Inter-Tenant mail flow and how it should be solved using custom transport agent. I have Exchange 2010 SP2 with ABPs in my lab now and Inter-Tenant mail flow works fine without them, but guidance document says that there could be problems if tenant is off-boarded. This would then brake the replies to e-mails exchanged beetwen tenants because messages will be sent to LegacyEchangeDN and not to SMTP address.
This custom transport agent should force all messages to be routed to the external SMTP relay server and back in to the Exchange Hub server so the messages appear as being external (and eliminate the LegacyExchangeDN issue I suppose).
I tried doing this with Send Connector to send all messages through the external SMTP but Exchange ignores that because it sees this as internal SMTP domain and e-mails are routed within Exchange hub servers.
Has anyone wrote such custom transport agent or are there commercially available ones? Is there some point that I'm missing here? Is possible future tenant off-boarding the only problem or are there any others?