emails are tracked in CRM unknowingly

We are using CRM 365 on-premise V9. I found some of users' emails are tracked in CRM. They are not emails responding emails from CRM. I am not sure how the system selecting emails to create activities.

More system info:

Email option for user in Personal Option is set to : Email messages in response to Dynamic 365 email.

Email option for user's queue is set to : Email messages in response to Dynamic 365 email.

Our system users all have a corresponding contact record in the system too. (not sure if it is relevant)

Any help? Thanks in advance!

Holly Li