SBS/Exchange 2000 and Yahoo.com email problems

Suddenly about a month ago, we started having problems sending emails to 
Yahoo.com addresses.  These are typically followups to telephone inquiries

that we get. The intended recipients complained about never getting the 
emails. We have had a few problems with Pacific Bell, AT&T, and SBC
emails. 
We are not having this problem with outbound emails to other domains.

I set up a test Yahoo email address and found out that test emails were 
getting placed into the spam/bulk folder. I tracked a few lost emails 
(through Exchange System Manager) and found that they were all delivered
to 
a Yahoo.com server, sometimes after a few hours of retries. My guess is
that 
Yahoo is falsely labelling our emails as suspected spam for some reason.
My 
further guess is that the Yahoo.com recipients do not know enough to look
in 
their spam folders and do not know enough to figure out if they told Yahoo

Mail to discard suspected spam rather than place it in their spam folder.

Our SBS 2000 server sends the emails directly under our domain name. We
have 
a fixed IP. I checked the IP against the spam blacklists and we were
clean. 
We have an SPF record and Google Mail says we pass the SPF test. We do not

have anything set up for DomainKeys -- I am not sure that we could with 
SBS/Exchange 2000.

Has anyone else experienced this problem? Any solutions? Any ideas for 
further diagnosis?

Thanks,

Joe M