Best offline email client for Gmail users

Postbox is sort of what comes after Eudora. I wanted an offline email reader that could

1) Use offline. This was before Gmail had that option.
2) Sync on more than one computer. So I use Postbox to read my mail on my MacBook Air when I travel, and if I delete something on my Air, it deletes on my desktop, etc.
3) Continue using SpamSieve since I have trained it so well for 10 years.
4) Interface with Gmail.

My mail goes through Gmail first and then SpamSieve so I have zero spam. One spam per week gets through and once every month I will check my sieve for one maybe two false positives.

Also, Postbox does threaded conversations (unlike Eudora) so it’s easier to track a conversation in email. It has a lot of other functions which I have not learned to use even though I have been using it for 3 years.

I can always read my mail in gmail if I want to go on the web.

-- KK 11/21/13

(I've been using Postbox since Kevin told me about it. It's the first email client to woo me away from using Gmail's web interface. I am impressed with its speed and its excellent integration with DropBox, Evernote, and Google Calendars. (Kevin and I can't vouch for the Windows version as we are Mac users.) - Mark Frauenfelder — editors)