Lenny, domains that use SPF could the problem. Check out this thread:
https://portal.smartertools.com/community/a520/email-forwarded-out-of-smartermail-failing-spf.aspx
Also, Bruce has a good KB article that explains how it all works and how to enable SRS to fix it:
https://portal.chicagonettech.com/kb/a177/enable-srs-to-prevent-spf-fail-on-forwarded-e-mail-messages.aspx