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
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.
Things that I think could be improved:
* There is no calendar feature
* No tasks feature
* It can occasionally get confused about SMTP cloud email and not allow an undo of a delete to work properly
Postbox allows me to consolidate multiple email accounts and organize, manage, and archive key emails locally as needed.