What is the Linux counterpart to MS Exchange?

Milti-part answer: Fist off, there is no really good Outlook/Exchange clone yet, due to MS's inability to correctly implement their own MAPI protocol :) Are you asking about Exchange or Outlook? Linux Mail servers commonly used are Sendmail, Qmail, etc. These do not have calandering (etc.) capabilities. There are new project out there that are more Exchange-like. Kroupware, Samsung Contact, etc. If anyone knows of better programs I'd love to hear about them. If you mean an Outlook like mail client, look to Xiamen's (sp?) Evolution. It has a lot of potential, and is just as bloated as Outlook. It also has a connector which will make it work with an MS Excange server. KMail can also be nice, if you're using a POP3/IMAP mailbox.