Note that the link above is to an FTP directory. Their website notes that the library has a proven track record from field testing and is perfect for developing email, Usenet or web applications. It also supports SMTP, POP and NNTP protocols, multipart messages, messages with attachments. You can customize the class or extend it through inheritance and polymorphism. Free for non-commercial, educational and personal use.
Source code is available.
It has abstractions for threads, sockets and synchronization objects. It supports Unix systems. The Linux platform is supported. This code is intended for the Windows platform. The source code here is short, so it is easy to read and understand, and will help you get started in understanding how to code for Winsock.