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. 

When an email is sent 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.

The issue occurs when the email address of the sender is not populated beside the display name in FROM field inside the activity log of a record

Activity log in a record:

Incoming email log of above record:

This behaviour is due to the mail server of the sender which is actually not populating the email address in the FROM field of the initial email.

The mail address is not populated in the email body under the FROM field on the instance and neither in the activity log of the respective record.

Reach your email admin and also check your outlook or email client properties to validate if the email address is actually updated in the "email" field on your email account profile.