We have unsubscribe forms that contain just an email address field. If someone inputs an email address that is not already on our database, Marketo will create a new lead on the database. We would like to stop this and validate the email address at the time that the unsubscribe form is submitted. Therefore, if the email address is not found on the database, it will display a message to the user and not submit the form until they enter an email that is on our database.
Is there an easy way to do this?
This is a form of the Lead Lookup Pattern and yes, it is, for the most part, possible.
However, even with it properly implemented (via some sleek JS) you can't prevent the new lead from existing for a moment in Marketo. You can delete it promptly, because any lead that gets created via the lookup process can be easily identified (via trigger constraints) and dumped.
Just ensure that whoever looks into this does *not* even consider using the Marketo REST API. That's emphatically not what I'm referring to, and is extremely dangerous to use in this public-facing context.