Trouble Sending Email After Installing PowerConnect 2716 Switch

PowerConnect 2716 , Mac OSX network

We're running a small (5 workstation) network. Just replaced a Netgear 8-port gigabit switch with a Dell PowerConnect 2716 16-port gigabit switch. Since the upgrade we are experiencing problems sending email. This is not isolated to any one computer, it is across the office. Nothing else in the system has been changed. We are NOT running the switch in managed mode. When a user attempts to send an email they immediately receive a message that says that the sending address was rejected by the server (at&t). It seems like the switch is delaying the transmission of the password used to authenticate SMTP. If the user keeps on sending the email it will eventually go, but it takes 3, 4, 5 or more attempts before it goes. Any idea what's up? Thanks.