As Outlook and Outlook on the web (by Microsoft design) inserts the email signature before the email address, you cannot automatically differentiate between internal/external emails.

However, if a special email signature for inter-office email is required, the Xink administrator simply makes it optional for those that need it and users can r-click on the signature and they can select it:

Alternatively, select all (ctrl+a) and begin typing when you don't want to use a signature.