Unibox is a people-centric email client that organizes your messages by person. It avoids having email like a to-do list. Unibox supports IMAP servers:
Gmail
iCloud, me.com, mac.com
Yahoo!
Exchange if IMAP is enabled
Hotmail, outlook.com, live.com
Fastmail
QQMail
Self-hosted IMAP servers
Many more
Unibox Mac App Store Apps
Single window: Instantly compose your messages without opening new windows. Conversations: Easily drill down into a conversation view with multiple participants.
Unibox Mac App Store App
Powerful Filters: Show only messages that are unread, starred, have attachments or were sent by people from your address book. Unified Accounts: View messages and conversations across all your accounts. Switch Accounts:
Unibox Mac App Store Settings
Focus on single accounts or folders when necessary. Attachment List: Quickly find documents and files you exchanged with other people.
Attachment Grid: Visually browse your attachments and images. Quicklook: Preview attachments without launching external apps. Aliases: Use multiple identities with a single account, including separate SMTP servers.
Unibox Mac App Store App Store
Btw, Unibox is now also available for iPhone and iPad.