If you are building a web mail application or any mail application that gets its data from web forms, etc, you can use the HTTPS protocol so that the data can not be intercepted as it travels between the user's browser and the server. In this situation the mail client is actually a combination of the browser and web server. HTTPS (SSL encrypted HTTP) protects data between the browser and the web server, and the web server process can also use SSL over SMTP, POP3, IMAP4, etc... to communicate with the actual mail server, if necessary.
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