Drop down field, send e-mail on selection.

Drop down field, send e-mail on selection.

Hi,

We have a form for members to submit a request to become a member. One of the fields is a drop down with status that the admin uses for approving a member.

Status:
1. pending (default)
2. approve
3. inactive

When the status changes to "Approve" we would like the system to send a welcome e-mail to the user.

How can we do this?

Thank you.
Sami