As far as Malte's proposal goes ... having these improvements in acs-mail-lite would be good but it's beyond the feature freeze date and ISTM we should wait until 5.1 ...
I think Malte's original TIP is a good idea, but the future of mail, combining two mail packages, changing the bad api and updating the rest of OpenACS and dotLRN (forgetting client code) in one TIP is a little much. When someone says "I approve" to this TIP, what does it mean?