Support Multipart-Related:, and the associated verbiage. ... But, like, who cares?
Complete multibyte support. Currently when we truncate sender names/subjects, we don't take into account multibyte characters. When glibc can provide more functionality (a'la mbnextc, mbprevc), I'll implement it.
Translate HTML into another language. Hey, multi-lingual support is already here, just dump your stuff under html, and make sure your browser sends me the right Accept Language tag.