spec/mail/network/retriever_methods/imap_spec.rb looks like the right place for the specs.

What's your use-case for this?

I was unable to connect to my IMAP-Server without using starttls. I always received the error message OpenSSL::SSL::SSLError: SSL_connect returned=1 errno=0 state=SSLv2/v3 read server hello A: unknown protocol when using the enable_ssl-option. Only with starttls enabled, connection works.

Regarding the tests: How should it be tested? I don't see any test for the enable_ssl-option, either. Can you give me a hint in the right direction for a test?

Until this option is merged, chances are that you can use imaps via port 993, then no STARTTLS is needed.

http://wiki.dovecot.org/SSL:

Originally SSL support was added to protocols by giving them a separate "SSL port" (imaps, pop3s, etc.), where the SSL handshake starts immediately when client connects, and only after the session is encrypted the regular protocol handling begins. Using two separate ports for plaintext and SSL connections was thought to be wasteful and adds complexity for clients which may wish to make use of SSL when it is advertised, so STARTTLS command was added and intended to deprecate the SSL ports. Clients using STARTTLS work by connecting to the regular unencrypted port and immediately issue a STARTTLS command, after which the session is encrypted. After SSL handshake there is no difference between SSL port initiated connections and STARTTLS initiated connections.

retriever_method :imap, :address    => "mail.your-mail-server",
                              :user_name  => 'your user name',
                              :password   => '***************',
                              :enable_ssl => true,
                              :port => 993