Can't send email from Exchange 2010 to Exchange 2007 in same domain - coexistence problem

We are in the co-existence stage of our Exchange 2007 to Exchange 2010 migration and after installing and configuring Exchange 2010, we moved one mailbox form Exchange 2007 to Exchange 2010 and created a new Exchange 2010 mailbox. Our new server has multiple roles (CAS, HUB, MBX) and we added Forefront TMG 2010 to protect OWA. We use a 3rd party smart host to protect from spam and viruses. We checked send / receive connectors and DNS records and all looks good. Email traffic flows find Internal > External, External > Internal, Excahnge 2007 to Exchange 2010. It fails from Exchange 2010 > Exchange 2007.

When I send the test email from the Exchange 2010 mailbox to the Exchange 2007 mailbox, the message gets stuck in the queue on the Exchange 2010 server at hub version 8 which is SMTP relay in Active Directory Site. There is an error message for the hub version 8 queue which says:
451 4.4.0 Primary target IP address responded with: "421 4.4.2 Connection dropped due to SocketError." Attempted failover to alternate host, but that did not succeed. Either there are no alternate hosts, or delivery failed to all alternate hosts.I seem to have the same symptoms as the one in this blog post but I am not sure where to go next.
Network Steve - Exchange 2010 and 2007 co-existence (hub transport)

Do I need to configure receive or send connectors for Exchange 2010 to Exchange 2007?

Telnet, ping, and tracert to legacy server form Exchange 2010 server are successful.

220 Ex2007server.internal.org Microsoft ESMTP MAIL Service ready at Wed, 11 Apr 20
12 15:42:41 -0600
EHLO mail.company.com
250-Ex2007server.internal.org Hello [<IP address of Ex2010Server>]
250-SIZE
250-PIPELINING
250-DSN
250-ENHANCEDSTATUSCODES
250-X-ANONYMOUSTLS
250-AUTH NTLM
250-X-EXPS GSSAPI NTLM
250-8BITMIME
250-BINARYMIME
250-CHUNKING
250-XEXCH50
250 XRDST
mail from:Ex2010mbx_user@company.com
250 2.1.0 Sender OK
rcpt to:Ex2007mbx_user@company.com notify=success,failure
250 2.1.5 Recipient OK
data
354 Start mail input; end with <CRLF>.<CRLF>
Subject: Test from usoemail
This is a test message from Johnny
.
250 2.6.0 <b2734f8f-8ad5-4317-9668-46a04eb5439a@Ex2007Server.internal.org> Queued
mail for delivery
quit
221 2.0.0 Service closing transmission channel

Thank you for your help.
USOEAsked:
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USOEAuthor Commented:
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ash007Commented:
Can you create a new send connector and try to send mail
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USOEAuthor Commented:
I had to call Microsoft for support and it was well worth it. It ended up being a registry entry that had to be made on the Exchange Server to get the mail flowing from Exhcange 2010 to Exchange 2007 in the organization. Here are the steps:
1)      We had  mail flow issue between Exchange server 2007 and 2010
2)      We removed the unwanted exchange certificate.
3)      Then we added a registry entry in the below location and value.

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\SecurityProviders\SCHANNEL

Value name: SendTrustedIssuerList

Value type: REG_DWORD

Value data: 0 (False)

4)      Restarted Transport service on both Exchange servers.
5)      Mail flow started working.
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USOEAuthor Commented:
I solved the issue after purchasing a Microsoft Suport incident.
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