[fetchmail]Fetchmail and SpamAssassin

I'm installing an email service using sendmail, spamassassin, clamav and
mailscanner. I want to convert our current hosted email from our ISP to
a backup. 

My problem: I have been unable to get him to domain forward any email
caught while our primary server is down. I keep getting the run-around.
In reviewing Fetchmail it looks like this is an ideal solution. I can
use it to fetch any email caught by my ISP and SMTP forward it to our
sendmail server. 

My question: Will Fetchmail reduce the spam filtering capabilities of
SpamAssassin? If so, how? For example, one of the things the
SpamAssassin does is it check for SMTP forgeries (e.g., a forged SMTP
HELO). It looks like Fetchmail delivers email as "localhost" (although
it has an invisible mode) so I curious is will any spammer information
be lost because the email has already been received by the upstream
email server?

                                                Thanks, Ken