Exchange 2003 & Exchg. 5.5

Presently bringing on 2003 exch. with exchg. 5.5 Use scanmail for virus protection. We also use PKI cert’s for sending mail. While acct’s are still on the 5.5 server, mail all moves through 2003 exchg. and out via connectors. When sent w/certificate from 5.5 we get a delivery failed, recip unknown. We have shut off scan mail and get same result. If message is sent from 2003 exchange w/cert, it goes fine. If sent from 5.5 account without cert. it goes fine. Problem is when sent from 5.5 exchange w/pki certificate, it fails. Has anyone seen this and/or suggestions to correct. Its not virus protection but how mail with certificate is handled between 2 servers.