Our Global Address List is managed by the ITW O365 team in the US and this is where you would go to add an ITW employee contact card to your contact list. 

Instructions attached: 

  - Add contact from Outlook Global Address List

The issue here is: 

  - The contact card was modified and Outlook started to update all the old emails to the new address. This is not desirable as it will cause a conflict.

  - The best approach is to NOT modify the ITW contact card for that person when they leave. 

Then go into your Outlook contacts and manually create a new one for that person with their external email address.

Now you should have two contact card; one for the person with their ITW contact details and one with their new contact details. 

Instructions attached: 

  - Manually add a contact to Outlook