DNS Issue with one client periodically

dmw

I have an issue with one client that has an imap connection to an Exchange 2003 server. Periodically sending mail will stop working and it seems as if its starts to resolve to an old mail server that has been removed in the past month. Periodically meaning it works in the morning then breaks a few hours later then is fixed if a dns flush is done or reboot. This is one of a few clients that are running using imap most are using cached exchange mode. This particular user is using a mozilla mail client. I have looked in DNS and do not see the record it is resolving to. I am wondering if this could be somewhere on the clients setting and where it would be?