Exchane 2007 to exchang e 2013 migration mail flow issue

We have recently migrated our  Exchange server 2007 to exchange server 2013. We have configured DAG and CAS array in exchange server 2013.Our migration has been done successfully and now all is working fine from exchange server 2013.

our issue is the outlook client is asking for a password frequently thought it can configured successfully, Still we have not done the decommission of our exchange server 2007.
Our scenario is as below
Ex2007 with mailbox, hub and client access role
1st Exch2013 with mailbox and client access
2nd Exch2013 with mailbox and client access

We have tested our scenario as below,
Now ex2007 is just shutdown,
when both new exchange server 2013 are on everything is working fine,
if we shutdown 2nd exchange server 2013 then also we are able to access owa and mailbox from 1st exchange server  also in this case mailflow is working fine. all is working fine from 1st exchange server.

if we shutdown the 1st exchange server 2013 then again we are able to access owa and mailbox from 2nd exchange server but in this case we are only able to send the internal emails. Also we can send email to the external clients but we are not able to receive emails from the external clients.
When we again start the 1st exchange server then all emails are came automatically and mailflow is working fine.

based on that I assume that there is something wrong with our 2nd ex2013 server which we need to fix and we should be able to receive emails from the external clients also when 1st exchange server is down.

Kindly suggest for the both issue
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MikeIT ManagerCommented:
Make sure all the DNS records are pointing to the IP for your CAS array and not your first exchange server.  Since you can use OWA and you can see mailboxes your DAG and CAS array is configured properly.

As far as your other problem, make sure the clients are connecting to your new CAS array and not your old 2007 server by doing CTRL+Right Click>Connection Status on the Outlook icon in the system tray.
Will SzymkowskiSenior Solution ArchitectCommented:
You need to make sure that in order to receive email from the internet through the firewall that your IP of your load balancer is properly NAT'ed through the firewall. What i suspect is that your IP of Ex2013 server 1 has its IP address added in the firewall and Ex2013 server 2 IP address is not.


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rigelnetAuthor Commented:
We have point the both server with MX record and that settings are seems to be right. Also we have done above mail flow testing by shutdown of exchange server 2007.

Do we need to upgrade the Cumulative updates cu8 which is latest?
Kindly suggest,
rigelnetAuthor Commented:
thank you
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