Their email is usually part of the contact information that you get from IDEA. At least it always has been for contacts that I have gotten. Then I email them through my business email. A few have been phishing emails, so beware. I never open any attachments from contacts that I haven’t met in person. And even then, I try to keep all information either in the body of emails or on paper when it comes to people I just met.
If you log into your account and go to “My Account” and find the “Quick Links” section on the left-hand side of the page, you’ll see an option that says “My Clients & Leads.” If you click on that and then click on the person’s name who contacted you, you’ll find that person’s email. Once you have that you can email that person directly.
Hope that helps!