Notes is the client, Domino is the server (in case you want to Google some of the details). Domino CAN provide webmail, POP mail, and IMAP mail. Whether they're available in your case depends on the people who run your server.
The Notes client is available on the Mac, too. 7.x is current, but there is also a very different version (based on Eclipse) in beta - 8.5, and it's available for download from IBM. It's a beta, but it works perfectly well for me. It requires 10.5, though.