essexboy has it right.
If you receive your Yahoo mail via POP3 then avast can scan it and can append the clean message (that's why I asked curly67 the question earlier in this thread).
You can get Yahoo mail delivered by POP3:
1) if you pay for it
2) if your Yahoo domain still offers free POP3 access
3) you use a program to convert Yahoo Webmail to POP3 (YPops, MrPostman, FreePops etc) or if you use certain email programs that have a built in converter for Yahoo (Hotmail as well) like Incredimail & Foxmail.