How to collect email addresses of people that click the 'Send Enquiry' button

I would like to be able to collect the email addresses of people the click on the ‘Send Enquiry’ button so that I can follow-up with them about their experience with the businesses that they enquired about. I would like to send them an email (or better, add them to an email drip campaign on MailChimp) that encourages them to write reviews of businesses they engage with. This is a core part of a business plan.

I’ve checked the database of a site running GeoDirectory, and the only reference to the email address I can see is in post type postman_sent_mail.

Is there any way to collect these addresses other than copying and pasting from the content of an email?

The site I’m hoping to do this with will be running GD version 2.