Ranked in these QuestionsQuestion Ranking
Pro Free and open source
Pro Drag & drop support
Roundcube supports attaching files simply by dragging and dropping them in.
Pro Sophisticated address book
Each contact can have their e-mail address, phone number, location, photos and associated URLs added to them. You can then group contacts and address your mails to the group instead of having to enter each person separately. Additionally, you can import vCards and CSV files as contact information. And, it has LDAP connectors.
Pro Built-in caching
Roundcube caches messages to allow quicker access to your mailbox and to alleviate the strain put on the server. You can turn off caching (change 'enable_caching' property in your config/main.inc.php file) if you need messages to appear instantly.
Pro Plug-in support
Roundcube is available in over 70 languages.
Pro Access control support
Roundcube has built-in support for access control lists (ACL).
Has auto-refresh functionality for email boxes.
Pro Good documentation
You have access to the documentation for APIs as well plugins development.
Pro Spell checking
You can enable/disable spellcheck by changing "enable_spellcheck" in main.inc.php file.
Not easy to create your own themes.