I do kind of think it is an E-mail issue of some sort. It has always worked in the past.
I would say it is 'more' than likely to be a filter, you might have yours off, but your ISP as Vladimyr said might also operate a spam filter. Check if they do operate spam filtering and if so have them allow avast.com or if they need to be more specific, register @ avast.com (without the spaces).
Try using a different domain email address, if you haven't got one you could get a free Yahoo one, which has worked in the past to receive the registration key.