Pegasus Mail is a free, standards-based, electronic mail client suitable for use by single or multiple users on single computers or in local area network environments (and also integrates well into Novell's NetWare in both Bindery and Directory Services modes). A proven product, serving millions of users for almost two decades, it is extremely feature-rich and powerful, yet remains small and fast.
HTML eMail messages, which are often used by spammers to track user activity (this is commonly viewed as an invasion of privacy), can contain references to external files (such as pictures). Pegasus Mail never attempts to download external files when viewing HTML eMails, thus by way of this mechanism spammers can't determine if you've ever viewed their spam.
*** IMPORTANT NOTE ***
The Lumber Cartel wholeheartedly and enthusiastically approves of Pegasus Mail because, in addition to explicitly prohibiting spamming in the licensing terms and conditions, the explanation that "...spam is obviously a social problem, not a technical one..." is included in the documentation, which indicates that the author - David Harris - understands the underlying fundamental nuances of the efforts faced by spam-fighters in counteracting the global spam problem.