It is as said "potentially" dangerous.
Depending on which email-program you use, i-frames can be used to execute arbitrary code. Doesn't mean every i-frame has to be dangerous, but it carries a risk in it. An i-frame-exploit is an exploit contained within a body of HTML and is typically used in email. The exploit allows malicious code to be executed by reading or previewing files containing this code. This exploit deceives the HTML viewer into executing viral code that the viewer thinks is an Audio or a Video component of the body of HTML.
Depending on what email-program you use, the mailprogram uses microsofts viewer (usually internet-explorer) to view the i-frame. This is a security risk which I personally would NOT take.
Only if you use an absolutely safe mail-program like iScribe with its own safe viewer, you might consider taking the risk, but if I were you, I'd delete it.
