I am thinking of creating a mail client as a side project and wanted a brief overview of how they work. A few issues I was thinking of are:
How do they connect to SMTP servers (My guess is by using built in libraries)?
Where are mails stored?
How are passwords encrypted?
How do they check if computer is connected to internet?
Your question is too broad to answer here. But you may check the following titles to get an initial idea.
First of all you should learn how the SMTP protocol works. It is old and stable, so this is easy to manage. You may start by reading [wikipedia's SMTP title] (http://en.wikipedia.org/wiki/Simple_Mail_Transfer_Protocol) . In fact there exists a full SMTP transport example, which you can try with a working smtp server after establishing a telnet connection.
Also, as you want to implement a mail client, you should learn about IMAP and POP3 by reading related wikipedia titles and its references.
