There are email replacement apps, but unfortunately the few that act similar to HTC Mail will cost money.
Touchdown by Nitrodesk: Touchdown is my preferred email replacement as I use the exchange functionality, and imap protocol. It can tie into most major email services, and also offers some premium features for business users.
Enhanced Email: A more powerful revamp of the stock Email app that is starting to become popular. It offers some premium features and the developer is very responsive if you have run into issues.
Moxier Mail (free trial): Offers support for all major protocols, its user interface is probably the closest to the HTC Mailapp.
Maildroid (free): Never used MailDroid, but it is yet another email replacement. The upside is there is a free Ad supported version.
Hope those help!