During migration of Exchange 2007 deployment with 3 AD sites to Exchange 2013, two site non Internet facing and one site is Internet facing, there are following settings for ActiveSync:

Two Exchange 2007 CAS in Internet facing AD site

[PS] C:\Documents and Settings\administrator>Get-ActiveSyncVirtualDirectory -Ser
ver CAS2007-01 | fl InternalURL, ExternalURL, *AuthenticationMethods*

InternalUrl : https://CAS2007-01.contoso.local/Microsoft-Server-A
ctiveSync
ExternalUrl :
InternalAuthenticationMethods : {}
ExternalAuthenticationMethods : {}

[PS] C:\Documents and Settings\administrator>Get-ActiveSyncVirtualDirectory -Ser
ver CAS2007-02 | fl InternalURL, ExternalURL, *AuthenticationMethods*

InternalUrl : https://CAS2007-02.contoso.local/Microsoft-Server-A
ctiveSync
ExternalUrl :
InternalAuthenticationMethods : {}
ExternalAuthenticationMethods : {}

One Exchange 2007 CAS in non-Internet facing AD site:

[PS] C:\Documents and Settings\administrator>Get-ActiveSyncVirtualDirectory -Ser
ver CAS2007-03 | fl InternalURL, ExternalURL, *AuthenticationMethods*

InternalUrl : https://CAS2007-03.contoso.local/Microsoft-Server-A
ctiveSync
ExternalUrl :
InternalAuthenticationMethods : {}
ExternalAuthenticationMethods : {}

One Exchange 2007 CAS in the other non-Internet facing AD site:

[PS] C:\Documents and Settings\administrator>Get-ActiveSyncVirtualDirectory -Ser
ver CAS2007-04 | fl InternalURL, ExternalURL, *AuthenticationMethods*

InternalUrl : https://CAS2007-04.contoso.local/Microsoft-Server-A
ctiveSync
ExternalUrl :
InternalAuthenticationMethods : {}
ExternalAuthenticationMethods : {}

In coexistence between Exchange 2007 and Exchange 2013, two Exchange 2013 CAS were installed in Internet facing AD site and two Exchange 2013 CAS in others two non-Internet facing AD site

On Exchange 2013 CAS in Internet facing AD site the following settings were done on ActiveSync virtual directory

[PS] C:\Windows\system32>Get-ActiveSyncVirtualDirectory -Server CAS2013-01 | fl InternalURL,ExternalURL,*AuthenticationMe
thods*

InternalUrl : https://CAS2013-01.contoso.local/Microsoft-Server-ActiveSync
ExternalUrl : https://mail.contoso.com/Microsoft-Server-ActiveSync
InternalAuthenticationMethods : {}
ExternalAuthenticationMethods : {}

[PS] C:\Windows\system32>Get-ActiveSyncVirtualDirectory -Server CAS2013-02 | fl InternalURL,ExternalURL,*AuthenticationMe
thods*

InternalUrl : https://CAS2013-02.contoso.local/Microsoft-Server-ActiveSync
ExternalUrl : https://mail.contoso.com/Microsoft-Server-ActiveSync
InternalAuthenticationMethods : {}
ExternalAuthenticationMethods : {}

According to this setup, devices with mailbox on Exchange 2007 should be proxied by CAS 2013 to the CAS 2007 in the right AD site
Unfortunately, Exchange 2007 mailbox mobile phones receives 503 Http status codes and Http status codes 449.

Mailboxes on Exchange 2013 works well when configured on mobile devices through ActiveSync.

Please advise on the pr