Using Captive Portal to Collect Email Addresses

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I'm currently using a pfSense appliance in a hotel primarily for its captive portal feature, requiring guests to enter a username/ password for internet access. I have been asked to modify the landing page so, rather than entering a username, they enter an email address.

Does anyone have experience in setting up a landing page like this? How would the entered email addresses be stored, tested or even collected. I was thinking using phpmailer to send each login as it's entered, but I'm unsure of pfSense's capabilities.