Some update:
Some HTML messages are sent with subject in UTF-8 but with body in iso-8859-7,
the subject is readble but not the body. If specify to read everything as text the are all readable . . .
Finally found out that amavisd-new was messing up with the message headers, as all messages that was unreadable fwere found at /quarantine and amavisd-new had add an extra header :
BAD HEADER non-encoded 8bit
Now trying a workaround for this . .