If these are mails with html embedded from a site on the net, maybe the site got hacked/infected and therefore avast also reports the ‘old’ messages infected.
asyn
you can try forwarding one of the problem mails to avast at virus@avast.com so they can see
exsplain the problem…then you may get an answer with a solution…
There’s nothing to solve for you, if the mails are really infected… You could contact the sender of the messages and/or the admin of the site from where the embedded html comes from. If theses dedections should be FPs do as Pondus stated and send one of the mails to avast.