The Bat! has tags support and can use internal html viewer (faster but sometime not so accurate) or use the more compatible Windows Internet Explorer control (similar to Outlook).

The Bat! is a powerful email client, many functions and deep customizations, has a plugin system and a template language for automatically create standard replies.
Maybe the interface is not so "modern" but it's faster and the program do not require so much ram (generally, for me, is using between 70 and 130MB).
Crash resistant - sometime I've terminated it and no email is lost - and I've folder with several GB of email with thousand of email.
Maybe too much functions for a basic use and this can be a little confusing for a newbie that must explore many menu and options.