Cypht
All your E-mail, from all your accounts, in one place. Cypht is not your father's webmail. Unless you are one of my daughters, in which case it is your father's webmail. Cypht is like a news reader, but for E-mail. Cypht does not replace your existing accounts - it combines them into one. And it's also a news reader.
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The driving force behind Cypht development is to provide combined views for multiple accounts, but it's also a standard E-mail client that lets you browse and manage IMAP folders (or POP3 inboxes), and send outbound messages with SMTP.
Cypht is an application built entirely of plugins, or as we call them, module sets (which is obviously way cooler sounding than plugins), that are executed by the framework. Modules provide a flexible way to add new features or customize the program without hacking the code.
Installation instructions https://cypht.org/install.html
Security details: https://cypht.org/security.html
Module info: https://cypht.org/modules.html
Features: https://cypht.org/features.html
License: https://cypht.org/license.html
Community chat room: https://gitter.im/cypht-org/community
For developers, get via Composer: https://packagist.org/packages/jason-munro/cypht
Contact: jason@cypht.org