How to check if an email address is already in my database.

How to check if an email address is already in my database.

I want to write a script that takes the information from my donation field and checks the email address against the contributor database.  If they are not in the list, I want it to send an email to the address with a form for them to fill out and enter their information.  I want to make a comparison like this:

if (input.email not in contributor.email)
(
send email
)

the problem is that I can't figure out how to reference the contributor form from the donation form.  The underlined reference does not work.

Thanks