Running WP 5.6 and Constant Contact Forms for WordPress 1.10.1
The email syntax verification does not include top level domain verification and as such you can enter an email in a format such as this:
and it will allow this non conforming email to be passed.
Thank you for reaching out to Constant Contact API Developer Support and for your patience. My team is here to assist outside software developers with questions about building into Constant Contact's API.
It looks like we've already followed up with you directly via email, but I wanted to close the loop here as well. If you haven't received our reply, please let us know via email at firstname.lastname@example.org, and reference case number 27879122.