I think you mean suspicious email, avast usually says why it is suspicious. Check your avast Log Viewer, right click the avast icon and select it. Check the Warning section for more information on the alert/s and let us know.
If it is suspicious and avast alerts it should give you options, one is to ignore/do nothing, etc. that will let it through if you identify it as from your friend. This is covered by the Heuristics element of the Internet Mail provider and there are many reasons why an email might be considered suspicious, no subject, too much white space in the subject, etc.