Postbox has steadily improved over the years and I feel it is the best email client in the market for an installed email application. Things I like: * Handles an enormous amount of emails and searches them fast * Easy to transfer between computers and backup/restore * Supports multiple accounts including POP3 and SMTP * Clean User Interface and great usability * Highlighting rules to color message based on specific filters * Auto-folder or delete messages based on a filter * Quick reply within the body of the email * Cross-platform - works great on Windows or Mac * Autothreading I initially tried it because my Outlook application was getting super slow and couldn't handle my email archive history. I gave Postbox a try and it had a nice import from Outlook function which made things seemless. The support is good and they have been responsive anytime I have any issues.

Ability to manage several email accounts

The ability to manage numerous email accounts on different settings with no delay in sync.