the iFrame tag is more frequently used in web pages to import content into a specific area, this can also have code executed so it is a powerful tool which can/could be used for malicious purposes also. In emails it is more frequently used to import adverts, etc.
Check out the Internet Mail, Customize.., Heuristics, Permitted URLs and add the remote location (see image). You can't define specific email addresses only the domain name that the content is coming from.