Secondary email server

[Vincent Britton]

I realize this is a bit off topic but this is my last hope.  I figure that someone else out there has done this and can help me out;-)   My primary Email server is an Exchange server.  I would like to set a secondary/backup Email server for our domains.  I would like to set this up on a Linux machine running Sendmail, Qmail, Postfix, or whatever!  I can't seem to find any documentation on how to set this configuration up!  I have the 1000 page Sendmail book from Orielly and looked a equally sized Qmail book last night.  They both say that it is a good Idea but neither say how to set it up!   Does anyone else out there have this configuration or know of a good source for documentation for this?   I know I can do this with exchange and if money was no object I would set up another!  But I don't have those kinda resources!   List Charter and FAQ at: http://www.sunbelt-software.com/exchange_list_charter.htm