Mail server for small business

[David.Grudek@anixter.com]

I maintain a lot of small businesses that are not large enough to have 
their own mail server hosted internally.  I would like to have a mail 
server that runs internally and fetches the mail from the providers and 
then host it locally.  The provider does not support IMAP and the users 
space is only 10 megs and if we had this pull the mail and then host it 
internally then we could have as much space as we wanted.  It would also 
be nice if we could pull multiple email addresses even with different 
domains and put them into one common mail file on this server, incase a 
user has a yahoo account as well.  Each user would need to pull their own 
mail to the own box on the server.    If there is a tool for this, could 
some one let me know.  I have a lot of Microsoft stuff to do this but this 
would be a nice step towards getting rid of them.