Very common and very know topic for New Exchange Admins.
By default each e-mail address has one default policy and it is setup when you configure your Exchange Server. So if you setup your Exchange with Contoso.local it will setup as yourname@contoso.local and this will become your REPLY Address. What happens is that at recipients side when they hot reply it will be repling to .local address not to the .com and you will never receive the reply. In order to fix it un-check the Automatically upgrade e-mail address based on e-mail address policy and you are all set.