i need help for sendmail configuration in our linux machine.

Here the things: I want to send email to outside by using our exchange server as the mail relay.But when sending the email through the server,it will response "user unknown".To make it worse, it will bounce back all the sent message to my localhost.

I already tested our configuration by using external mail server such as gmail and yahoo,the configuration is working without any issue and the email can be sent to the recipient.Most of the configuration of my sendmail is based on here.

authinfo file :

AuthInfo:my_exchange_server "U:my_name" "I:my_email" "P:my_passwd" "M:PLAIN LOGIN"
AuthInfo:my_exchange_server:587 "U:my_name" "I:my_email" "P:my_passwd" "M:PLAIN LOGIN"

sendmail.mc :

FEATURE(authinfo,hash /etc/mail/authinfo.db)
define(`SMART_HOST', `my_exchange server')dnl
define('RELAY_MAILER_ARGS', 'TCP $h 587')
define('ESMTP_MAILER_ARGS', 'TCP $h 587')
define('confCACERT_PATH', '/usr/share/ssl/certs')
define('confCACET','/usr/share/ssl/certs/ca-bundle.crt')
define('confSERVER_CERT','/usr/share/ssl/certs/sendmail.pem')
define('confSERVER_KEY','/usr/share/ssl/certs/sendmail.pem')
define('confAUTH_MECHANISMS', 'EXTERNAL GSSAPI DIGEST-MD5 CRAM-MD5 LOGIN PLAIN')
TRUST_AUTH_MECH('EXTERNAL DIGEST-MD5 CRAM-MD5 LOGIN PLAIN')
define('confAUTH_OPTIONS, 'A')dnl

My first assumptions the problem occur is due to the authentication problem, as exchange server need encrypted authentication (DIGEST-MD5).I have already changed this in the authinfo file (from plain login to digest-md5 login) but still not working.

I also can telnet our exchange server.So the port is not being blocked by firewall.

Can someone help me out with this problems?I'm really at wits ends. Thanks.