I have been starting to setup a EX2007->2013 migration.

And after pointing the clients to the new 2013 frontend clients are starting to get a login prompt in Outlook. 

And the reason must be that according to the exchange deployment assistant you should set basic authentication for outlook anywehere. 

So my setup is:

ExternalClientAuthenticationMethod : Basic
InternalClientAuthenticationMethod : Ntlm
IISAuthenticationMethods           : {Basic, Ntlm}
Server                             : 2007legacy

ExternalClientAuthenticationMethod : Ntlm
InternalClientAuthenticationMethod : Ntlm
IISAuthenticationMethods           : {Basic, Ntlm, Negotiate}

Server: 2013frontend

http://o365info.com/exchange-2013-coexistence-environment-and-outlook-infrastructure-part-22/

http://silbers.net/blog/2014/01/22/exchange-20072013-coexistence-urls/

What happens if we change to NTLM on the 2007 as well?

A note to remember is that we dont have any internalclients.

HI

There should not be any issue, bcz 2007 Support NTLM for outlook anywhere, only thing you firewall should supports NTLM auth.

Thanks & Regards

Zahoor

• Proposed as answer by Niko.ChengMicrosoft contingent staff, Moderator a few seconds ago