Step 10 below will exclude internal emails and step 11/12 will stop the footer from being added over and over again.
- Sign into mail.office365.com with a user account that has admin rights
- Click OPTIONS in the top right corner of Outlook Web App (OWA)
- Click SEE ALL OPTIONS
- Use the MAIL > OPTIONS menu at the top left to change from MANAGE MYSELF to MY ORGANIZATION. If you don’t see this option, you are not signed in with an Admin account.
- Click MAIL CONTROL in the menu on the left
- Click RULES (although you are likely defaulted their already)
- Click the NEW button
- Select: If THE SENDER IS IN THE SCOPE OF and set it to INSIDE THE ORAGANIZATION
- Click the ADD CONDITION button
- Select THE RECIPIENT IS IN THE SCOPE OF and set it to OUTSIDE THE ORGANIZATION
- in DO THE FOLLOWING, select APPEND A DISCLAIMER TO THE MESSAGE, type in your footer text and set the fallback action to IGNORE
- Copy some unique part of the footer text
- in EXCEPT IF select THE SUBJECT OR BODY MATCHES and paste in the unique part of the footer text (from step 12)
- name your rule and click the SAVE button at the bottom.
For more detail see THIS Microsoft article