No security program have 100% detection.
I dont use avast, but to my knowledge avast dont have a spam filter (this was removed some years ago)
So for avast to detect this i guess A: the attachment must be a malicious file. B: attachment must contain a clickable link that is on avast blacklist
Your first line of defence should be your mail providers spam/virus filter (how did they bypass that)
Log inn to your mail using webmail and mark mail as spam so the filter learn and send them to spam folder