The domain detection is based on the URL the end user hits, there is no configurable setting.
For example, if you client reaches their mail server at http://mail.customersdomain121212.com
and enter the username as 'Joe' the account that's authenticating is technically email@example.com.
If the customer hits mail.customerdomain151515.com and attempts to sign in as 'Joe' the account will attempt to authenticate as firstname.lastname@example.org.
Please note that this functionality will not work with POP\IMAP clients, but should work with EWS\EAS protocols in addition to the web interface as long as the e-mail client sends the appropriate connection information based on the server entered in the connection settings.
Bindings\Hostnames are related to this functionality only in regards to what hostnames are setup to listen within your IIS bindings.
I hope this helps clarify.
Technical Support Supervisor