IFrame Not Showing in an Email
Issue
After adding an IFrame to an email in the Marketo email editor, the IFrame seems to disappear when you view the email in an email client (such as Outlook, Apple Mail, or Gmail).
Solution
The reason why you can see the IFrame in Marketo but not in your email client is because not all email clients support IFrames. If yours does not support them, the IFrame will be blocked and will not appear in the email. You can try viewing the email in another email client to see the IFrame.