Feb 22 23:04:39 gthill sendmail: k1MEwfge022073: timeout waiting for input from mx3.mail.yahoo.com. during client greeting
Is your server behind a router, on a DSL line with a dynamic IP address? It's likely that big email providers block these IP addresses. Another reason might be that your server doesn't have a proper PTR or SPF record.
Please contact Yahoo and ask them about their policies regarding the acceptance of emails.