This project that I am working on is giving me one gold nugget after another... Anyway, so we are trying to setup co-existence between Exchange 2013 and 2007, and we stumbled into this issue.
Exchange 2013 does not seem want to proxy ActiveSync request for users still having mailbox on Exchange 2007. When testing Exchange Connectivity Test (https://testconnectivity.microsoft.com/), the error returned was:
An HTTP 500 response was returned from IIS7.
It seems that it's Exchange 2007 at fault here. The only way to fix it is to recreate the ActiveSync virtual directory on Exchange 2007, using the following PowerShell commands:
Remove-ActiveSyncVirtualDirectory -Identity "EXCHANGEServer\Microsoft-Server-ActiveSync (Default Web Site)" -Confirm:$false
New-ActiveSyncVirtualDirectory -WebsiteName "Default Web Site" -InternalUrl "https://webmail.company.com/Microsoft-Server-ActiveSync" -ExternalAuthenticationMethods Basic -InternalAuthenticationMethods Basic
The Exchange 2007 Microsoft-Server-ActiveSync virtual directory must be corrupted along the way.