An extensible desktop mail app built on the modern web
...In early 2016, the Nylas Mail team wrote extensive documentation for the app that was intended for plugin developers. This documentation lives on GitHub Pages and offers a great overview of the app's architecture and important classes. When you download and build Nylas Mail from the source it runs without its cloud components. The concept of a "Nylas ID" / subscription has been removed, and plugins that require server-side processing are disabled by default.
Gmail Notifier (restartless) is an open-source project that notify you about incoming emails from all your Google Mail accounts and labels.
Features:
1. No requirement to enter your credentials.
2. Multiple account support
3. Multiple label support
4. Low bandwidth usage by using RSS technology
5. Mark as read, report span, trash or archive massages right from toolbar
GNUsTicker is basically a RSS aggregator. It is a GNOME panel applet that scrolls infos retrieved from RSS, RDF, etc. It may be extended for feeding from different sources (an example for POP3 mailserver is provided).
Mobile Agent Server and Controller(MASC) - is a server, controller and viewer for aglets, written in Java, with an attractive and user-friendly GUI .MASC is easy to install and configure.It also provides mail notification,schedule,aglet class browser,etc.
Auth0 Token Vault handles secure token storage, exchange, and refresh for external providers so you don't have to build it yourself.
Rolling your own OAuth token storage can be a security liability. Token Vault securely stores access and refresh tokens from federated providers and handles exchange and renewal automatically. Connected accounts, refresh exchange, and privileged worker flows included.
gbiff checks for mail within a file, in a qmail or MH style dir, or on an IMAP4, POP3 or APOP server, and can display headers (number, sender, subject, and date) when new mail has arrived. It also interprets any ISO-8859 encoding and supports both GTK an
Java-WebAdmin is a web-enabled tool that supports dynamic modules, Modules are developed independent from the framwork. Examples are File uploading/editing, server administration, e-mail practicly anything can be done!
Cronos II is a MUA (or Mail Client) written from scratch over the GNOME library using C. This MUA is intended to be fast, light and powerful. It has support for POP3, Spool, SMTP, Plugins and many other protocols and features.