I have a workflow that triggers an email to a customer after an estimate is in an approved state. This works fine, however, I would like to change the "From Field" on the Sender parameter to be one that is sourced from a customer email address field.

The custom email address field that is available on the estimate record, and is populated with a valid email address, is not available as a selection in the Field dropdown when the record is set to "Current Record".

Any direction on what makes a field available to chose as the Sender on the current record would be very appreciated!