Outlook Issue

The last few days users are telling me that they are experiancing an Outlook problem. When they send an email, the email remains in the outbox even after doing a send/receive. I’ve tested it myself and the message will sit in the outbox upwards of 8 minutes. Now I tried sending an email to my yahoo.com account and the same probably occurred, but in reality I was actually receiving emails at the yahoo account but the email was still saying it was in the outbox in Outlook. I’ve contacted my ISP who handles our email and there is nothing wrong on their end. I was advised that it’s a client side issue. My users range from Outlook 2003 to Outlook 2007. Any help or suggestions on this would be great.

-Kevin