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The whole webmail client can be carried around on your external device (such as a USB stick) and used without having admin access to install.
Pro Free and open source
Licensed under AGPL.
Pro Beautiful UI
Uncluttered, highly intuitive, easy to use user interface.
Pro Built-in rock-solid and easy-to-use encryption
Mailpile offers support for OpenPGP and S/MIME encryption and signatures in an easy to use manner.
Pro Good documentation
Pro Fast, scalable search engine
The search engine is at the very center of Mailpile. It was specifically designed to be able to handle large amount of e-mails without grinding the machine to a halt.
Additionally, the search includes a tagging feature that can automatically organize new mail by set rules or bayesian classifiers.
Pro Plugin architecture
Pro Internationalization support
Mailpile is available in over 40 languages.
Pro Theme support
The underlying architecture of Mailpile allows for themes that not only change the visual aesthetic of the client, but layout as well allowing for different UX designs.
Con It is actually more of an email client with a web interface than a complete solution with mail in a cloud
You need access to the device you install it on. So it is not really 'webmail'.