I have a very very strange problem with Exchange 2003. It goes as follow: Persons that try to send email to valid recepient here receive a "550 (status 5.1.1) mailbox unavailable" error message. BUT, 1) The problem does not always occur. Sometimes it works, sometimes it doesn't. It can work 4 time with the same message, than fail 2 times, then succeed again 2 times. 2) It doesn't depends on attachment or not. The same message can have an attachment or not, it will not affect if it fails or not. 3) SMTP log for the transaction shows that commands are issued correctly up to the RCPT command, where Exchange respond with a 550. 4) For some recepient on the server, this never happen. 5) For some recepient on the server, it happen a LOT. 6) For some recepient on the server, it happens sometimes, but not that much. 7) It seems to be related to the remote SMTP, but logs show that there's nothing special about the SMTP transaction. And it happens with several different remote SMTP server. Hereis the SMTP log when it succeed: 2007-07-04 20:37:22 209.226.175.107 tomts33-srv.bellnexxia.net SMTPSVC1 SERVER-1 192.168.0.5 0 EHLO - +tomts33-srv.bellnexxia.net 250 SMTP -2007-07-04 20:37:22 209.226.175.107 tomts33-srv.bellnexxia.net SMTPSVC1 SERVER-1 192.168.0.5 0 MAIL - +FROM:+jdriver@xxxx.xxx 250 SMTP -2007-07-04 20:37:22 209.226.175.107 tomts33-srv.bellnexxia.net SMTPSVC1 SERVER-1 192.168.0.5 0 RCPT - +TO:+daniel.malouin@xxxx.xxx 250 SMTP -2007-07-04 20:37:22 209.226.175.107 tomts33-srv.bellnexxia.net SMTPSVC1 SERVER-1 192.168.0.5 0 RCPT - +TO:+etienne.fortin@xxxx.xxx 250 SMTP -2007-07-04 20:37:23 209.226.175.107 tomts33-srv.bellnexxia.net SMTPSVC1 SERVER-1 192.168.0.5 0 DATA - +<004701c7be7b$246c7150$6d4553f0$@com> 250 SMTP -2007-07-04 20:37:23 209.226.175.107 tomts33-srv.bellnexxia.net SMTPSVC1 SERVER-1 192.168.0.5 0 QUIT - tomts33-srv.bellnexxia.net 240 SMTP - Here is the SMTP log when it fails: 2007-07-04 20:38:13 209.226.175.101 tomts27-srv.bellnexxia.net SMTPSVC1 SERVER-1 192.168.0.5 0 EHLO - +tomts27-srv.bellnexxia.net 250 SMTP -2007-07-04 20:38:13 209.226.175.101 tomts27-srv.bellnexxia.net SMTPSVC1 SERVER-1 192.168.0.5 0 MAIL - +FROM:+jdriver@xxxx.xxx 250 SMTP -2007-07-04 20:38:13 209.226.175.101 tomts27-srv.bellnexxia.net SMTPSVC1 SERVER-1 192.168.0.5 0 RCPT - +TO:+daniel.malouin@xxxx.xxx 550 SMTP -2007-07-04 20:38:13 209.226.175.101 tomts27-srv.bellnexxia.net SMTPSVC1 SERVER-1 192.168.0.5 0 RCPT - +TO:+etienne.fortin@xxxx.xxx 550 SMTP -2007-07-04 20:38:13 209.226.175.101 tomts27-srv.bellnexxia.net SMTPSVC1 SERVER-1 192.168.0.5 0 QUIT - tomts27-srv.bellnexxia.net 240 SMTP - Will my Exchange 2003 explode in the near future??? I fear the worst! Please help.

I wonder if it's a problem between Exchange and Active Directory. Is there any reported connection problem between Exchange and AD? What error code would be sent to the remote SMTP server if Exchange is unable to connect to AD to identify the recepient specified with the RCPT command? Would that be a 550? How can I log the transaction between Exchange and AD?