SMTP stop working

I am using Cpanel/WHM and the mailer is exim.
Some users cannotnot send email(Others can).
They could do that before yesterday.
The error is like this:

550-Host 120.2.111.219.st.excite.co.jp ([192.168.1.20]) [219.111.2.120] is
not permitted
550-to relay through server02.joeswebhosting.net.
550-Perhaps you have not logged into the pop/imap server in the last 30
minutes.
550-You may also have been rejected because your ip address
550-does not have a reverse DNS entry.

Anybody who experienced the same?
Can you tell me how to fix it?
Thank you.

 

 

 

 

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