build campaign for referrals

Hello, I am building a campaign flow, and need to find a way to trigger an email to refrees' on referral accounts. So the break down is, a referral comes in through a webform, the referral will have the contact and account that referred them in the system. once the referral become an active account i need an email to trigger to the contact that referred them in the first place. How do i set up an email to trigger to that referral