I've not heard of such a situation reported to us previously. It sounds to me like a browser extension that's detecting an email address being entered and pulling up the mail client.
Here's a few questions I thought of that might shed more light on your situation:
Is this happening for all users or just some?
Does the browser (IE/FF/Chrome) make any difference?
What about OS?
Does this happen when the user is typing it or using copy/paste (or some other input method entirely)?
I have not seen this happen before and I just tried this in my 2017.2.1 version. I am able to type in the email address and either hit return or tab, etc, and my default mail program does not open. Only time it opens is if I click on the actual email address after already entered in the HTML field.
You may want to log a case with Ivanti support for this if it becomes problematic.