I don’t know why you find this so odd, as you already stated other businesses also don’t accept your email redirections for security reasons…I guess Avast also see it in the same light.

I’ve no issue using an Outlook account and their security is provided by Trendmicro I believe, I’ve also not seen an incident such as yours before here on the Avast forum so I’m fairly sure that your unusual email setup is the reason for your problem.