KMail

KMail is the email component of Kontact, the integrated personal information manager from KDE.

Secure

Secure Default. Default settings rather compromise on features than security or privacy.
End-to-end Encryption. Support for OpenPGP and S/MIME is built-in, and a setup and key generation wizard aids users in getting started with this.
Transport Encryption. SSL/TLS encrypted communication is of course supported, as are a number of authentication methods such as GSSAPI (Kerberos) or OAuth2.
Strong isolation of HTML content. If HTML emails can't be avoided, KMail ensures no external references can be hidden in there to leak information or compromise your privacy. Additionally KMail's phishing protections warns about suspicious links in such emails.
Spam Protection. If your email server doesn't take care of this already, KMail can integrate popular spam checkers such as SpamAssassin or Bogofilter locally.

Offline Capable. KMail allows you to operate entirely without network connectivity by (optionally) syncing your entire email locally.
Multiple Identities. KMail separates identities from accounts, giving you a lot of flexibility when wearing different hats in multiple organizations.
Templates. A flexible template and text snippet system support you in automating parts of your email writing.
Multi-lingual. Built-in content translation support and per-paragraph spell checking language auto-detection support you when dealing with content in multiple languages.
Productive Composer. KMail's email composer comes with many small helpers to make you more efficient, such as warning about forgotten attachments, inline attachment compression or resizing of attached images.
Powerful Filtering. Besides automating the sorting of your emails by a multitude conditions, KMail's filtering system can also be used for implementing arbitrary workflows as it can integrate with external applications.
Searching and Tagging. Flagging and tagging of messages aid in sorting and information recovery, and so do the powerful local and remote searching capabilities.
Mailing List Management. Mailman mailing lists can be auto-detected and common mailing list management operations are directly available from within KMail.
Flexible Configuration. Countless configurations options allow you to tweak many aspects of KMail to exactly your needs and wishes.

iTip Invitations. Meeting invitation emails are detected and offer in-place quick responses, and are of course fed into KOrganizer.
Addressbook. Besides auto-completing address from KAddressBook, KMail also uses avatars and crypto preferences from there.
Booking Emails. KMail can detect flight or hotel reservations in emails from certain providers and offers to add them to your calendar.