XMail Server Lite is an open source & free mail server with an Ajaxed webmail bundled to it.
It is compatible with Window & Linux OSs with 2 different installers provided.

The server is capable of managing SMTP/ESMTP & POP3 protocols with web-based administration panel.
It supports unlimited domains/users including virtual domains & aliases.
XMail Lite also offers anti-virus integration, bayesian filters (for spam protection) & a logging functionality of all the activities.
The Webmail Lite can display folders/nested folders, has an address book, multilingual & much more.
Tip: The webmail can be downloaded & installed as a standalone product too (from here).
Requirements: Windows or Linux OS
Swift Mailer is a feature-rich PHP mailer library for sending e-mails from PHP websites and applications.
It does not rely on PHP’s native mail() class which consumes high resources. Instead, Swift communicates directly with an SMTP server or a MTA binary to send mail quickly and efficiently.
Swift supports event-driven plugins which offer you the opportunity to really take control of the library and set this mailer apart from everything else that’s out there.
Some features of Swift Mailer:
- Persistent connectivity improves performance
- Internationalization support (i18n)
- Load balancing and/or throttling support
- SSL & TLS Support – for Gmail servers
- Embedded images or other file types
- Full MIME 1.0 library included (create multipart messages, attachments etc)
- Batch mail processing
- Send attachments of any size even with PHP’s 8MB Memory Limit
- Support for multiple attachments
- Lossless protection against header injection (encode, don’t strip)
- Set message priority
- Request read receipts
- Pluggable SMTP authentication (LOGIN, PLAIN, MD5-CRAM, POP Before SMTP)
- Anti-flooding support for servers with limits on emails-per-connection
- Bandwidth monitor included
Compatibility: Compatibility PHP 4 & PHP 5