network analyse tool? To debug IMAP related problems

[张韡武]

Hello. I would wish to have a tool that would do this kind of thing:

     1. listen on imap port on localhost, connect to localhost with my
        email client;
     2. forward the traffic from/to/between real imap server;
     3. meanwhile, print everything being transfer-ed, so that I can
        have a good ovewview of server-client conversation;

I don't know what such kind of tool is usually called and thus difficult
to do an effective google search. I tried a few tools in ports/net but
none of them seems to be working in this way... (admit that I didn't
look into pkg-descr of every package)