We have an Exchange infrastructure going through a migration from 2003 SP2 (call it ExOld) to 2010 (ExNew).

All users are now on the new server, but mail is still being directed to ExOld until testing is complete. ExNew sends emails directly to the internet.

For one particular user, emails don't seem to be being reliably delivered, but the odd thing is that it's not all emails. I can see external emails in his inbox.

• If I send an internal email it works fine.
• If I send an email from Gmail to him it doesn't get through.
• If I telnet from outside to ExOld I can send an email to him.
• If I telnet from outside to ExNew I can send an email to him.

This is a transcript that results in a successful send:

    220 ExOldName Microsoft ESMTP MAIL Service, Version: 6.0.3790.4675 ready at  Mon, 22 Oct 2012 10:55:26 +0100
    EHLO test.com
    500 5.3.3 Unrecognized command
    EHLO test.com
    250-ExOldFQDN Hello [MyTestExternalIp]
    250-TURN
    250-SIZE
    250-ETRN
    250-PIPELINING
    250-DSN
    250-ENHANCEDSTATUSCODES
    250-8bitmime
    250-BINARYMIME
    250-CHUNKING
    250-VRFY
    250-X-EXPS GSSAPI NTLM LOGIN
    250-X-EXPS=LOGIN
    250-AUTH GSSAPI NTLM LOGIN
    250-AUTH=LOGIN
    250-X-LINK2STATE
    250-XEXCH50
    250 OK
    MAIL FROM:[email protected]
    250 2.1.0 [email protected] OK
    RCPT TO:[email protected] notify=success,failure
    250 2.1.5 [email protected] 
    DATA
    354 Start mail input; end with .
    Subject:Test 1056
    Test 10:56
    .
    250 2.6.0  Queued mail for delivery
    quit
    221 2.0.0 ExOldFQDN Service closing transmission channel

Emails go through Symantec Cloud, but their "Track and Trace" shows the messages going through, with a "delivered ok" log entry.

2012-10-22 09:19:56 Connection from: 209.85.212.171 (mail-wi0-f171.google.com)
2012-10-22 09:19:56 Sending server HELO string:mail-wi0-f171.google.com
2012-10-22 09:19:56 Message id:CAE5-_4hzGpY2kXFbzxu7gzEUSj5BAvi+BB5q1Gjb6UUOXOWT3g@mail.gmail.com
2012-10-22 09:19:56 Message reference: 135089759500000177171130001194006
2012-10-22 09:19:56 Sender: [email protected]
2012-10-22 09:19:56 Recipient: [email protected]
2012-10-22 09:20:26 SMTP Status: OK
2012-10-22 09:19:56 Delivery attempt #1 (final)
2012-10-22 09:19:56 Recipient server: ExOldIP (ExOldIP)
2012-10-22 09:19:56 Response: 250 2.6.0 Queued mail for delivery

I'm not sure where to look on the old (or new) server for information as to where the mails are ending up.