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In admin panel you can supply separate logos for login screen and for top-left corner of main interface.
Pro Multiple accounts
In WebMail Pro each user can have multiple accounts added to switch between them and use them separately.
Pro Sync with Windows, iOS, Android etc.
Offers syncing through DAV with mobile devices, there are also mobile apps and Windows sync client.
Pro External services integration
Twitter, Facebook, Google and Dropbox are supported. With last two, you can access their file storage; and you can use social account credentials to log into your WebMail acct.
Pro Plugins infrastructure
A few dozens of plugins, some of those created by community. Plugins are placed into data directory so they survive upgrades easily.
Pro Mobile version of interface
Open WebMail on your iOS or Android device to get a simplified mobile interface. And when run on iOS, it'll offer you to get iOS profile with settings for mail access, contacts and calendar sync.
Pro Multilanguage, with autodetection
Interface is available in over 30 languages. WebMail attempts to determine your system language and open the interface in it.
Pro Drag 'N Drop
Mails can be dragged into a different folder, contacts can be added into group, events in calendars can be tossed around, etc. - all with drag 'n drop.
Configurable autoreload of email message list every N minutes
Pro Available in multiple editions
In addition to commercial WebMail Pro, simplified free opensource WebMail Lite is available. Also there are MailSuite Pro (which is basically WebMail Pro integrated with mailserver software) and Aurora, extended with groupware features.
Con No Unified inbox
I think they planned it for a while, but for now, it's still working with one account at a time.
Con No CardDAV support in free version
CardDav Sync is not available in AfterLogic WebMail Lite, which is the free version.