Outlook clnt fails receive inbound mails

Hi, something strange is going on. Who has seen this behaviour in Outlook before ?
Environment : Exchange 5.5 – NT4 server ; Outlook 2000 clients – Windows 2000;
Scenario : all inbound traffic (no LAN problems detected)
Outlook client A sends a mail with the delivery option set to client B ; client B does not receive the mail however client A gets notified by the

exchange server that its mail has been delivered.
Client B sends a mail to A => no problem ; A receives the message
Client B can receive outbound mails wo a problem

=> what is going on that B cannot receive inbound mails ?

rgds,