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Roundcube is available in over 70 languages.
Pro Free and open source
Licensed under GPL v3 with code available on GitHub.
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 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 Plug-in support
Pro Drag & drop support
Roundcube supports attaching files simply by dragging and dropping them in.
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.
Con Extremely clunky unintuitive interface
For example the mouse cursor may start getting stuck and lag.
Con You can't make multiple accounts
It is not possible to make more then one account.
Con Search option are limited
You can only search for one item at a time instead of multiple search terms, which makes it difficult to go through large amounts of emails.
Unfortunately, all interfaces of roundcube look very old. I hope there will be a progress soon, Google Mail and Outlook Webmail should be the role models here. But also Yahoo mail offers an intuitive design.
If this open source software should survive, it must be completely renewed and bring up to new standards.