I recently setup a mail server for a company. I was blamed for using the same domain name for internal and external access. I was asked to use a different internal and external domain name instead. The irony is the difference between the domain names is so subtle. For external domain name, I was asked to use example.com and example.com.my for internal domain name.
The reason some people prefer using different names for internal and external domains is a DNS suffix like .corp can not be resolved by external network clients because it is not a publicly available top level domain name. The disadvantage is that this configuration requires you to manage two separate namespaces. Also, using a stand-alone internal domain that is unrelated to your external domain might create confusion for users because the namespaces do not reflect a relationship between resources within and outside of your network. Whatever security advantages that may be gained by this DNS design are mitigated by the complications and user dissatisfaction you’ll encounter when using different domain names for internal and external network resources.