well it depends upon how you use it.
postbox has connectivity with facebook and twitter.
it has interfaces showing all attachments in a single screen... so you can actually see all the attached files in one go without searching through mails...
it also has threaded mode similar to gmail which works very nicely. so if you are a gmail fan, this feature comes very handy.
search is also pretty powerful...
🙂 more here.
Meet Postbox — Postbox
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the only problem which is very major is that the final version 1.0 is likely to be a paid one. they are considering making available an ad-supported freeware edition. that is likely to be a major turnoff.
but i have been rather happy with the beta releases. and if they do go paid with the first gold version, i would stick with the last beta till i find a better alternative.
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i really would love to see gmail doing something for attachments in emails... similar to this program.
just make available all the attachments in a more usable user interface... so that it is damn easy to find the one you want...
if any web based app can do that, its gmail.