Warning this could leave you vulnerable
This is the Internet Mail provider, Customize, Heuristics, iFrame items. Changing these settings would apply to all emails with an iFrame Tab, personally I would rather manually allow those that I know and delete this that I don't.
iFrames are used to import data (which can be executed) into the frame. This is commonly used in web pages but not so widely in emails (usually for Ad content, but could be malicious) and could leave you vulnerable to execution of data when you view the email.