Exchange's server-side filters are pretty weak, so I decided to work around them by writing a tool that will fix my inbox and filter mail appropriately so that any client I use to view mail with (OWA, whatever) has the same view with no client-local filters. It's likely/possible there's already a tool that does this; let's ignore that possibility for now.

Ruby comes with Net::IMAP, but it doesn't come with an authenticator that supports 'PLAIN' auth, so we have to provide one:

# Learned the 'PLAIN' expected format from imapsync.
class PlainAuthenticator
  def process(data)
    # Net::IMAP takes care of base64 encoding the result of this...
    return "#{@user}\0#{@user}\0#{@password}"
  end
  
  def initialize(user, password)
    @user = user
    @password = password
  end
end

Net::IMAP::add_authenticator('PLAIN', PlainAuthenticator)

Now that we have that, let's try connecting.

imap = Net::IMAP.new("exchange.example.com", "imaps", usessl=true)
imap.authenticate("PLAIN", user, passwd)

This fails, because Exchange's IMAP server ignores the RFC:

/usr/lib/ruby/1.8/net/imap.rb:3122:in `parse_error': unexpected token CRLF (expected SPACE) (Net::IMAP::ResponseParseError)
        from /usr/lib/ruby/1.8/net/imap.rb:2974:in `match'
        from /usr/lib/ruby/1.8/net/imap.rb:1959:in `continue_req'
        from /usr/lib/ruby/1.8/net/imap.rb:1946:in `response'
...

Expected a space, not a crlf. The failure is in continue_req, which expects what the RFC says:

continue_req    ::= "+" SPACE (resp_text / base64)

However, Exchange's IMAP server doesn't send a space after the plus. Great, let's fix that by overriding the continue_req method:

# Copied/modified from net/imap.rb, don't modify that file, put this
# in your own code to override the continue_req method
module Net
  class IMAP
    class ResponseParser
      def continue_req
        match(T_PLUS)
        #match(T_SPACE)   # Comment this line out to not expect a space.
        return ContinuationRequest.new(resp_text, @str)
      end
    end
  end
end

Once you've done that, everything else seems to work normally. I have only tested listing mail folders thus far, but the hacks above allow you to get this far.