Supports both multiple accounts from different servers / services simultaneously (both inbound and sending) as well as multiple identities. Incoming accounts have a default identity and it switches automatically to the correct identity depending on the mail folder you are in.

The integrated calendaring support means that incoming invitations can be accepted with a click and they get put into your calendar automatically.

Besides being able to connect to any IMAP, POP3 and SMTP server, you can also connect your Google and Facebook accounts and access Kolab and Tine servers natively.

Fully supports PGP and S-MIME with built-in certificate management. Encrypt, sign and verify emails easily and securely.

Searching for emails is simple with the search box in the main window which can scan through email bodies, email flags, recipients, etc. Lightening fast in recent versions.

Provides support for rich-text (HTML) mail editing, but kindly defaults to plain text. Also supports viewing HTML emails with several security features built-in.

There are numerous components that come with Kontact that augment the mail experience including contact management, todo lists, note takers and an RSS feed reader, among others.

Based on qt, and part of plasma suite, it integrates well with other KDE apps like kwallet, calendar, etc.

Kontact has been actively developed for over a decade, with components such as the mail and calender going back even further. Development continues to this day with an active and responsive open source community made up of both volunteers and paid developers behind it.

IMAP support is a strong point for Kontact, with offline mail and sync being transparent and automatic. Read and create mails on the go and everything syncs quickly and reliably when network connectivity is available.

Supports spam filtering, scam detection and adblock for HTML mails.

Can be driven almost entirely from the keyboard. Moving an email, for instance, is a simple matter of hitting 'm' and then typing to filter through the list of folders that appears.

Works on Linux/BSD (all desktop environments and window managers), Windows and Mac.

Has flexible and powerful options, from "traditional" conversation-style threading to organizing by date, by senders and receivers, .... paired with "keep replies in same folder" it is easy to track conversations over time.

Supports client and server-side filtering of mails by subject, recipients, mailing list, body text, etc.

You can make the Kontact UI look and function how you want. You can re-arrange the main interface, as well as make the list of mails show or hide almost anything you could need.

For enterprise deployments, having commercial support can be a big plus. Companies such as Kolab Systems provide professional support and consulting services for Kontact.