The default smtp address (the one which is printed in bold) must be able to receive email. During the wireless activation process a small email with an attachment "etp.dat" will be sent to the adress you enter in the blackberry handheld when you activate it. The mail will be immediately removed from the mailbox by the BES server.
To check if the email arrives in the mailbox:
stop all blackberry services, try an activation. Then should see an email within a minute.
If the email does not arrive:
The email adress the user typed in the activation window is wrong
Some spam filtering software might have bounced it
The user has no Blackberry Enterprise data plan.
Maybe, visit this url:
https://www.blackberry.com/besc/dashboard/
to check if an enterprise activation should be possible.
If the email arrives later then within 3 minutes, some antispam software is delaying it - then deactivate al filtering/delaying and try again an activation.
If you server is polling the email using a pop3 connection, an activation is still possible, however, you have to poll the mail manually every 30 seconds.