How to update a field in a Contact record when an email template/form is sent to contact?

How to update a field in a Contact record when an email template/form is sent to contact?

We send an important form to our clients using an email template. We have a pick-list field in the contact record that tracks the status of the form (Not Started, Sent, Received).  We are able to automatically update the status field to RECEIVED when the contact submits the form based on an auto-date field. 

I haven't been able to come up with a way to update the status field to SENT when the email template with the form link is sent. 

Does anyone have any suggestions for how we might do that?