Exchange 2007 server not sending to Exchange 2010

We are migrating from a single Exchange 2007 server to Exchange 2010.

I have a problem with mail flowing from accounts with a mailbox on a
2007 server to a test account with a mailbox on a 2010 server.

Here is the co-existence infrastructure:

Server A 2007 has 3 roles - CAS, HT, MBX  (was the only server)
Server B 2010 has 1 role - CAS  (will be the new client access server)
Server C 2010 has 2 roles - HT, MBX  (will eventually be the only mailbox server)

Here is my problem:
Accounts with mailboxes on server A cannot send an email to accounts with
mailboxes on server C. They get an NDR report.  The queue viewer shows
the mail is being held in the queue on A.  However, the test user with
a mailbox on server C 2010 can send email to users with mailboxes on server A.  

Internet inbound mail has already been switched over to Server C from
an outside anti-spam provider and mail is being delivered to accounts
with mailboxes on server A 2007 and the test account mailbox on server C 2010.  

According to support documents I have read, when you install 2010 into a
2007 environment, no special connector is needed between the two mailbox
servers as long as the hub transport role is on each mailbox server.

However, when we first cut over Internet inbound mail to server C, mail would not
flow to mailboxes on server A. We had a narrow maintenance window to cut
over, so to fix this I created an Exchange send connector to route mail
from C to A with the selection "route mail through the following smart hosts"
checked for the address space * and designating A as the smart host.
Now mailboxes on both server A & C are receiving Internet mail.

We are preparing to switch over client access to the new server B CAS
this weekend and I have been testing that with host records on some internal
workstations.  Everything with the CAS seems to be fine. Accounts with mailboxes
on 2007 can access mail through OWA and also through Outlook Anywhere.

The goal is to migrate all mailboxes from server A to C.  However, until I
get email flowing from accounts with mailboxes on A to accounts with mailboxes
on C, I really can't begin moving mailboxes.  It isn't practical to move
all mailboxes at once as there are approximately 1000 users in various remote

So I need some help figuring this out.

Attached are screenshots of the send connectors for the organization and shots
of the receive connectors for the internal HT.

One other thing.  The company's main email domain is listed as authoritative in
the accepted domains, but it is also isted in the remote domains.
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Mohammed TahirMicrosoft Exchange and O365 AdministratorCommented:
Hi aa-denver,

First ensure you are successfully able to telnet Exchanage 2010 server from Exchange 2007 server on port 25.

In Co-Existence scenario, for smooth mailflow between the exchange servers we need to create dedicated receive connector to receive emails from other server.

I saw you are using the Default receive connector to receive from the exchange 2007. So just add the IP address of Exchange 2007 server in Exchange 2010 HT (Server C ) receive connector under Network Tab. Restart the MS Exchange Transport service and try to send to account on Exchange 2010.


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Peter HutchisonSenior Network Systems SpecialistCommented:
On the Server-A Receive connector, make sure BOTH Server A's and Server C's IP address are added to ensure that SErver-A and Server-C can connect to it to allow mail to flow as it seems its blocking the other server to send messages.
aa-denverAuthor Commented:
Thanks to both cmsxpjh and Md_tahir21 for inputs.  Both were on the right track.

Yes, I was able to telnet on port 25 from server A to server C, sent email via command-line and server C delivered to test mailbox.

Md_tahir's suggestion to explicitly put the address of server A in server C's default receive connector was the fix.   This is odd to me because I already had in the receive mail from box.  However, it worked.  I added a line as suggested for and mail started flowing.  All the stuck emails to the test account emptied out of the queue on server A.
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