Basically by ignoring it and allowing it to pass.
This is the avast heuristics at work and is not an indication of a virus but of potential for harm.
The iFrame is normally used in internet web pages and is used to dynamically import data into the iFrame, this could include executing code.
It isn't commonly used in email, but some legitimate usage is to import adverts into your emails. However, for malicious use it could execute code, that is why I say ignore the warning if it refers to that one source (that you trust ?), rather than disable the check which could leave you vulnerable.
