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.
Emails: Sending Emails on Behalf of Others –
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.
Campaigns: Sending Emails on Behalf of Others –
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.