Your server's outgoing mail log (probably in /var/log) would have a record of the transaction between your server and the other mail server, or the relay your server uses. Mail servers log pretty much everything, even successful deliveries. If mail is going through, but being rejected, it may be because of anti spam measures, or header problems. We've done a lot of work on the mail component in Open Journal Systems recently, maybe some of those improvements are relevant to OCS. Let me know what you find.