Email address of the sender who has initiated the mail to instance is not visible in the "FROM" field of the activity log. As the emails can be forwarded to the instance , the body of the email should contain the email address in FROM field of the email.
Let's assume an email is sent by a sender to the instance in order to update a record on a table. This email updates the record and the same is logged in the activity log of the incident.
This KB will address why the email address of the sender is not populated beside the display name in FROM field inside the activity log of a record
The root cause of the issue is with the mail server of the sender which is actually not populating the email address in the FROM field of the initial email. This is due to this , mail address is not populated in the email body under the FROM field on the instance and the activity log of the respective record.
Please check with your mail admin and also your outlook properties to validate if the email address is actually updated in the "email" field on your Outlook profile.
Activity log in a record:
Incoming email log of above record: