avast! doesn't send out spam, how have you ascertained avast sent you this ?
What benefit would avast gain from spamming you, none, what purpose would it serve spamming its customers, none, in fact it would have the effect your showing, to drive them away, so there really is no reason for the to do it.
Headers in email can be forged, where does it say it came from ?
I get lots of spam, my anti-spam program takes care of it and some of it is like this, but avast isn't sending it. I have a number of email addresses that have never been notified to avast and for the most part to no one else.
There are many ways your virgin email address can end up on email spam lists. There are now many infected systems out there that act only as spam bots searching out email addresses. Many have their email address book and many other email addresses captured by an infected system and those addresses are transmitted to the spammers.
So if someone who you correspond with has an infected system it is entirely possible that your email will end up on these lists.