NeoMail vs Horde vs Squirrelmail

NeoMail, Horde and SquirrelMail are 3 commonly free webmail client offer by most hosting company (limited to linux hosts only). 

1.  NeoMail
   Neomail has been discontinued due to long-standing security and stability issues, and the fact that the largest control panel vendor, cPanel, has also discontinued support for Neomail
2.  Horde
   I find Horde mail client (Internet Messaging Program or the IMP) is the best, It is written in PHP and provides webmail access to IMAP and POP3 accounts. It is included with cPanel and Plesk installations as a webmail client. It often integrates email, calendar, address book, notes, tasks, filters and a newsreader with cPanel.Check out screenshots here.
3. Squirrelmail
   The SquirrelMail client itself is a complete webmail system, but extra features are available in the form of plugins. A plugin allows non-standard features to be added to SquirrelMail, often without the need to modify the source code. There are over 200 third-party plugins available for download from the SquirrelMail website, and SquirrelMail ships with several “standard” or “core” plugins, allowing an administrator to add:
   • Spell checking (squirrelspell)
   • Mail filters (filters)
   • Web-based administration of SquirrelMail (administrator)
   • A calendar (calendar)
   • An interface to submit bug reports semi-automatically (bug_report)