Emails will be sent from the user that is logged in by default, but you have the opportunity to select a specific user or the contact's sales rep if you have the appropriate permissions.

You will select the sender on the last step of the email wizard. Click the Change Sender button to select a different user.

Select the desired name from the drop-down list of users, or choose to send from the contact's sales rep, and Save.

Head to Tools > Campaigns and either edit an existing campaign or create a new one.

With the automation to Send an Email you will be able to select if the email should send from the contact's sales rep or a specific user.