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hi, <br />
can you please take a look on this KB, that can solve your issue , <br />
Llink <a href="https://support.microsoft.com/en-us/help/956531/outlook-2007-prompts-you-repeatedly-for-a-password-under-certain-netwo" rel="ugc">https://support.microsoft.com/en-us/help/956531/outlook-2007-prompts-you-repeatedly-for-a-password-under-certain-netwo</a>
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Do you have OutlookAnywhere enabled and working on your Exchange 2007?<br />
<br />
CAS Servers in Exchange 2013 is only a http proxy to the corresponding backend server, all the things that used to be in the CAS role in your Exchange 2007 are now in the mailbox role.<br />
<br />
You will have to keep an Exchange 2007 CAS server in your infrastructure for co-existence, and also need OutlookAnywhere enabled on the 2007 Exchange Server<br />
<br />
Also the authentification settings should be the same on Exchange 2007 and Exchange 2013. You can verify this with<br />
<br />

<pre><code class="code-1 nolinks" id="code-20-42729697-1"><span>Get-OutlookAnywhere | fl servername, externalclientauthenticationmethod, internalclientauthenticationmethod, iisauthenticationmethods</span>
</code></pre>
<br />
Maybe this Microsoft Blog article can also help you with your migration: <a href="https://blogs.technet.microsoft.com/rmilne/2014/03/19/exchange-2007-and-2013-outlook-anywhere-co-existence/" rel="ugc">https://blogs.technet.microsoft.com/rmilne/2014/03/19/exchange-2007-and-2013-outlook-anywhere-co-existence/</a>
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fulloutput

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<div class="content wysiwyg-content">
Hi<br />
<br />
I am currently in process of upgrading out exch2007 to exch2013. I have all clients connecting to my 2013 CAS server. 2013/owa users can login with no problem and access their mailbox. If the mailbox is on exch2007 it does not redirect to legacy to locate the mailbox (From internal and External). <br />
<br />
Ive even manually added host Files on local machine for testing as follow:<br />
<br />
192.168.1.4 exchange13.domain.local<br />
192.168.1.4 autodiscover.domain.local<br />
192.168.1.3 legacy.domain.local<br />
(just example) <br />
<br />
*Virtual Directories are setup in 2013 and legacy in 2007.<br />
2ndly the outlook cleint of a 2007 Mailbox keeps asking for authentication and never connects.
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</div>


Hi

I am currently in process of upgrading out exch2007 to exch2013. I have all clients connecting to my 2013 CAS server. 2013/owa users can login with no problem and access their mailbox. If the mailbox is on exch2007 it does not redirect to legacy to locate the mailbox (From internal and External). 

Ive even manually added host Files on local machine for testing as follow:

192.168.1.4 exchange13.domain.local
192.168.1.4 autodiscover.domain.local
192.168.1.3 legacy.domain.local
(just example) 

*Virtual Directories are setup in 2013 and legacy in 2007.
2ndly the outlook cleint of a 2007 Mailbox keeps asking for authentication and never connects.

CAS 2013 to Mailbox 2007 redirection

Exchange is the server side of a collaborative application product that is part of the Microsoft Server infrastructure. Exchange's major features include email, calendaring, contacts and tasks, support for mobile and web-based access to information, and support for data storage.