Xiant Mobile - Kindle Fire Edition (Android) app for $2.99
Supports Microsoft Exchange (with ActiveSync), Gmail, Gmail for Businesses, and IMAP. Best mail search for Android: search your entire mail history with one query. Sync mail from multiple accounts -- or use a combined inbox. Highlights names of important contacts found in each message. Supports folder sync, message starring/flagging, attachment handling, and calendar requests. Integrates contact information from Gmail and Microsoft Exchange accounts to your Kindle Fire's Contacts app.