To add additional security to email, ITSC will enable the External Tagging in Exchange Online for Outlook users staring from July 2021. External email warning helps to alert users from clicking malicious links in phishing emails sent by external senders. 

You will notice that there is an additional tag "External" or “From outside your organization” in messages from outsiders if you are using any of the following Outlook applications:

• Outlook on the web (via desktop web browser)
  
   
• Outlook for Windows: version 2211 or higher
  
  
  version 2107 or higher:
  
• Outlook mobile (iOS & Android): version 4.2111.0 and higher
  
   
• Outlook for Mac: version 16.47 and higher