linear - 6:34 pm on Aug 10, 2006 (gmt 0)

[antispam.yahoo.com...]

The exact criteria they use to inject the header aren't well-documented, so it's not possible for us to suggest a specific fix.

is there a way to tell yahoo that i am a legit non-spammer?

You can look into implementing domainkeys:
[antispam.yahoo.com...]

It's not trivial, and it's not entirely clear that it accomplishes your stated goal. Setting up a sendmail milter is not something many people can accomplish on their own. Even if you get it right, not every provider has adopted domainkeys, you'd probably need SPF records too.
[openspf.org...]