No email client that I'm aware of supports JS in email. (Thunderbird used to have a hidden option for turning it on, but even that was removed when we doubted the ability to enforce sandboxing at all). All email clients sanitize the HTML in email to some extent anyways.
Everything. iframes can't fuck with you code, but they are 1. displayed in your page (is this a good idea, does this iframe agree with how your page will display it). 2. Is leaking information to this iframe (and consequently, that iframe's server that this email had been opened) a good idea (no. 100% it's not.). 3. Can someone else contrive another vector of page control or information leak that suits their motives and hasn't been considered a priori by you (also yes, 100%. Never underestimate the motivation or creativity of others).
Never, ever, EVER embed an iframe thinking it will make your life better.