K-9 Mail is an open source email client for the Android operating system. It is designed to be lightweight and highly customizable, while still providing a powerful set of features. It supports IMAP, POP3, and Exchange 2003/2007 (with WebDAV). K-9 Mail allows users to manage multiple email accounts, download and view attachments, and organize their emails by thread, label, and folder. It also provides support for a wide range of email encryption protocols, including TLS, SSL, and PGP/MIME. Additionally, K-9 Mail allows users to customize their experience with a variety of themes and settings, such as font size, background color, and notification settings.
The discontinued Opera Mail is no longer updated and is no longer supported, but the latest version, 1.0.1044, released in February 2016, can still be downloaded from the official website.