To secure mail retrieval, SSL support must be enabled for dovecot. You need to put in dovecot configuration file references to your SSL certificate, SSL certificate key and SSL certification autority bundle.
The dovecot configuration file should include:
From the above example, the 'server.key' references your server's certificate key file, usually generated by you along with the certificate request. The other two files contains the 'CA Bundle', the certificates for the CA's on the certification path, and the server's certificate, provided by the certification authority. I haven't tested if the certificate key can be used encrypted, I always used an unencrypted form of the key file, secured by file system attributes. More info on what is a certification authority, a certificate request, a certificate key, etc, can be found on OpenSSL site and on certificate authority provider's web pages, like VeriSign, Thawte, Comodo, etc.
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