dsn=5.0.0, stat=Service unavailable

Hi expert，

I setup to use external SMTP server (Exchange server) to send e-mail on my Linux server. However, it gave me an error message"

 dsn=5.0.0, stat=Service unavailable

The command line: Echo "Test Email Message" | sendmail "Subeject Test" xxx@xxx.com

tail /var/log/maillog:

Jan  2 06:54:44 localhost sendmail[25472]: x026shcK025470: x026sicK025472: DSN: Service unavailable
Jan  2 06:54:44 localhost sendmail[25472]: x026sicK025472: to=<xxx@localhost.localdomain>, delay=00:00:00, xdelay=00:00:00,                                                                                                              mailer=local, pri=31521, dsn=2.0.0, stat=Sent
How to resolve this issue?

Thanks for any input.

Regards,

Pirlo Zhang 

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