connectivity issues with mail.app and jaguar server

Hey folks, Setup: 10.2.5 server running Apple Mail Services on a medium switched network. Recently, our users running Mail.app (10.2.6), even with static IPs, get the following message when sending mail SOMETIMES: "Unable to Connect. There may be a problem with the mail server or network. Check the settings for account or try again." HOWEVER, the message always sends anyway! Restarting the server doesn't help. Any thoughts? Gracias, Noam