Users not getting each other's emails

Hello,

I'm having experiencing a situation where a user in our organization (using outlook) cannot email an external user (hotmail) and viceversa.

Everybody in the organization can send and receive emails to and from this external user with the exemption of this one person in our organization.

Both our internal and the external user have checked their junk mail folders and both users have added each other's email addresses to their safe sender's list.

I ran a message trace in the Exchange Admin Console and there's no traffic to be found. 

Thank you in advance for your help