Hello all. 

We have Exchange 2013 CU9 infrastructure.

4 Servers in HQ, 1 Server (CAS+Mailbox colocated) in every Branch Office.

All domain users are located in one Active Directory domain.

Email addresses in HQ are @contoso.com, in branches users have addresses like @la.contoso.com, fl.contoso.com.

External DNS records: mail.contoso.com, autodiscover.contoso.com, autodiscover.la.contoso.com, autodiscover.fl.contoso.com.

Direct IP NAT is translated to one CAS Server in HQ.

Outlook Anywhere works fine for users in HQ with addresses like @contoso.com.

But branch office users with branch email addresses can not connect via Outlook Anywhere. If I add @contoso.com SMTP-address to a branch user, everything works. But we should avoid this situation.

External URL on all virtual catalogs in branches is set to $null.

Also we have a problem with autodiscover for activesync branch users.

Does everybody had such problem with multiple SMTP domains? Is it technically possible to implement such Exchange Topology?

Hi 

If you want the users of this  subdomains  @la.contoso.com to access services from externally then you need to create mx records for them and point them to your external MX ip

Hello, how is outlook anywhere connectivity related to mx records and Mail routing?

Can you run Test Outlook connectivity for this subdomain and see the results 

If you get any error can you paste the errors

It isn't.

If autodiscover.la.contoso.com points to the appropriate server, there's a certificate that matches that name bound to IIS, etc., then it should return the information for that site.  You could also point it to the main site and it should work that way too.

You didn't say whether the branches are Internet-facing or not; that makes a difference.

This is a very good document that explains how all of this should work.

http://blogs.technet.com/b/exchange/archive/2013/01/25/exchange-2013-client-access-server-role.aspx

And here's a video that might help.

https://channel9.msdn.com/Events/TechEd/NorthAmerica/2013/OUC-B313#fbid=