stdnoj is a cross-platform C++ class library that includes classes for using portable strings, databases, SMTP, POP, NNTP, arrays, files, directories, logging, email, and sockets. While templates are used, no reliance upon STL or other (traditionally error-prone) technologies has been used, which increases portability between compilers.
Twisted Mail provides client and server implementations of SMTP, POP3, and IMAP4. These differentiate themselves from the Python standard library implementations both by presenting a much higher-level, easy-to-use interface and in their server components, which allow the implementation of custom servers for each protocol without dealing with protocol-level issues. Twisted Mail includes a simple demonstration email server which accepts messages over SMTP, stores them in a Maildir arrangement, and can serve them to clients over POP3.