I belong to several groups, mainly under the Yahoo banner and use both a hotmail address and others with my ISP. Out of the blue in the last week I’ve received notices that messages to me and in one case one from me have been bounced and my Yahoo groupmembership deactivated. I have made no alterations to anything email or avast wise but wonder if there may be someting in the AV program that could be causing the difficulty.
I doubt it, avast doesn’t do anything to your email other than scan it for infection. Depending on your OS (and you don’t mention which it is) and/or email program (again no mention) your account settings may need to be customised (but this doesen’t appear to be the problem, as you wouldn’t be able to send or receive email if it wasn’t set-up correctly).
I would email Yahoo Groups support asking why this has happened and also visit their FAQ/Support pages and see if there is/are any problems like this already mentioned (I doubt that this is the first time this has happened).