Your Marketing Forms account is configured to send an email to users who attempt to submit your form if there's an error with their payment. For instance, if a user can enter the wrong credit card number and after the form displays a decline message from the payment gateway, a failure email is automatically sent to the user.
Here's what the standard failure form submit email looks like:
Tokens from your account will include your contact email and phone number as well as your default donation form URL (/join). This is a site-wide email but we can customize by form.