Internal Server not able to send email after changing which HubTransport server to use

We have several servers that send SMTP email internally. These emails are typically server status notices after a process is complete.
To prep for some DR tests, I changed the Exchange HubTransport server used to receive the emails into Exchange.
I double and triple checked both of the HubTransport servers had the exact same Receive connector.

After the change of the HubTransport server, mail flowed properly and I could see in the header that the new HubTransport server was used for a majority (more than 10) of the servers sending emails.

But we had 4 servers that received "5.7.1 Client was not Authenticated" and the mail did not flow.

Since this was during the day and it was hours after the change with still no mail flow on these problem servers, we had to change the HubTransport server BACK to the original one. When we did this 3 of them still did not work. We got a brainstorm and did a DNS Flush on the problem servers which corrected the issue.

With this information, we feel that we can put the new HubTransport server change back in and be prepared to just Flush the DNS on the problem servers and result in the email flowing from these servers.

Is anything I may have missed? If the Receive connector is the same on both HubTransport servers is there anything else that could have caused this issue?

This is an Exchange 2007 server environment on Win 2008 boxes.

Thank you,
swfwmd2Asked:
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Simon Butler (Sembee)Connect With a Mentor ConsultantCommented:
Are you sure the connectors are 100% identical, because from the error you have posted it would appear not.

run

get-receiveconnector | fl and then compare the output.

How are you controlling the server they use? DNS entry? If so, turn the TTL time down, so any changes are effective quickly. Running it with a 10 or 15 minute TTL internally wouldn't cause a problem.

Simon.
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