Unable to send Email

One of the workstations in our LAN has a curious problem. When we attempt to send email (using Outlook Express) from this computer, we get an error message (copied below). It LOOKS like an authentication problem, but I’ve triple- and quadruple-checked the settings and they’re correct. I’ve even imported/exported settings from other stations, with no luck. This does not happen on other workstations. Any ideas?

Error message is as follows:
The message could not be sent because one of the recipients was rejected by the server. The rejected e-mail address was ‘kyle@lewisartgallery.com’. Subject ‘test’, Account: ‘PTB’, Server: ‘pop.omah.qwest.net’, Protocol: SMTP, Server Response: ‘553 relaying denied.’, Port: 25, Secure(SSL): No, Server Error: 553, Error Number: 0x800CCC79