If you get an error when creating an EnFin proposal that states Invalid email address at partnerDetails.3122.email, check the customer's email.
The customer email must be valid, and entering a fake or invalid email address, will result in an error. EnFin does not accept placeholder email addresses such as no@gmail.com.
To resolve, click the three dots next to the customer's name.
Click into the customer tab and update the email.
Click Save.