How should a graduate admissions committee view (possibly false) accusations against an applicant? My question is about a situation that nobody wants to see but might have already happened in history (I have heard of this only from some second-hand sources). I ask this because there are people in here who are serving or have served on a graduate school admissions committee and would like to hear their thoughts. 

Suppose during the admissions process a department secretary or a professor on the committee receives some emails accusing an applicant of academic misconduct. In most cases, such letters could be forged by the writer (out of jealousy or enmity) and they probably don't come from official email addresses especially when then information is fake.

I wonder in practice how such letters will be regarded in the admission process. Should the committee inform the applicant of this and let them explain it or contact the institute of the applicant to verify or disprove the claims?
If I were the head of the grad admissions committee (which I have been in the past) and either an anonymous or signed but unofficial email came to me or a faculty member, I would:

1) if the email is from an email address that is reply-able (or from a person who is googleable), I would write to them and request they make an official statement. I would also double check whether the email is actually the one from the person it denotes. 

2) if the email is anonymous or the email address doesn’t match, I would instruct faculty to ignore it. 

3) however, in either case it’s my responsibility to inform the graduate school that we’ve received derogatory information of unknown provenance. The grad school will likely tell us to ignore it but I need to cover my ass. 

In reality, I’ve reviewed well over a thousand files in my decade plus of teaching at the graduate level and haven’t yet received one of these notes. Most faculty if they have something to say, are not shy about writing it in the main letter themselves (whether in coded form or outright).