Limiting mailbox to internal mail only

[Sultanofcyber]

I thought that I could stop a user from receiving e-mail from outside the company by removing their Internet e-mail address and leaving the CCMail, MS, and X400 addresses but today I was proven wrong when the receipient received e-mail from outside the company.
How do I configure a mailbox to receive only internal e-mail?

 

[brontosaurus]

May I ask how they received it? Via X.400 address?

 

[Sultanofcyber]

That is what I don't know. How do I find out? It looked as though it was addressed as a normal Internet address for example user@company.com

 

[brontosaurus]

check the message header (View, Options) from the email.

 

[Norwich]

If it's enabled, you could also use the message tracking tool or check the tracking logs