I'm confused with the Exchange Server 2007 Outlook Anywhere Setup and MS TMG 2010.

I have an environment with the following settings

Single TMG 2010 Server

- published rules for exchange Outlook Anywhere/ActiveSync/Outlook Web App to use an SSL listener.

The SSL listener uses a SSL certificate  that has subject CN as "XYZ.contoso.com", with SAN names that include mail.contoso.com

Single Exchange 2007 Server

- outlook anywhere, basic authentication, and external host name set to "XYZ.contoso.com"

Users access externally through mail.contoso.com site.

My question is it doesn't matter if I set the external host name field of the outlook anywhere configuration to "XYZ.contoso.com" or "mail.contoso.com", they will both work. Shouldn't it work only for mail.contoso.com or just XYZ.contoso.com?

Thanks in advance

In your case, TMG does most of the work with clients, and it communicates with Exchange instead of clients. That's why it works with both names.