All outgoing e-mails going into recipients Junk Mail folders.

I recently took over administration of a Company network. Almost all the
users reported that their sent e-mails were going into the receipients Junk
E-Mail folders, which of course was causing many problems.

DETAILS:
- I checked DNSStuff.org; their domain and mail server is NOT listed in
any spam database.
- They company was using an external hosted e-mail solution, and have now
switched to an internal mail server using Exchange 2003 SP2. Same problems
before and after the switch.
- The users are using Outlook 2003 SP3 connected to Exchange 2003 Sp2
(Small Business Server).

Any ideas what would cause an entire network's e-mails to be classified as
Junk E-Mail? Thanks,
MitchM