AtMail Open is described as 'AtMail is an open source webmail client written in PHP. We aim to provide a elegant Ajax webmail client for existing IMAP mailservers, with less bloat and a focus on an intuitive, simple user interface' and is an website in the Online Services category. There are more than 10 alternatives to AtMail Open, not only websites but also apps for a variety of platforms, including Self-Hosted solutions, Linux, Windows and Mac. The best alternative is Zoho Mail, which is free. Other great sites and apps similar to AtMail Open are Roundcube (Free, Open Source), Horde (Free, Open Source), Squirrelmail (Free, Open Source) and Webmail Lite (Free, Open Source).
Zoho Mail is a webmail service built with the needs of a modern business in mind. It provides features for business users to communicate effectively while catering to the customization and mailbox management needs of IT administrators.
Roundcube webmail is a browser-based multilingual IMAP client with an application-like user interface. It provides full functionality you expect from an e-mail client, including MIME support, address book, folder manipulation, message searching and spell checking.
Horde Groupware is a free, enterprise ready, browser based collaboration suite. Users can manage and share calendars, contacts, tasks and notes with the standards compliant components from the Horde Project.
Sendmail is a general purpose internetwork email routing facility that supports many kinds of transfer and delivery methods, including the Simple Mail Transfer Protocol (SMTP) used for email transport over the Internet.