Customer's email was blocked recently

Hi,

Recently one of customer received rejection messages for all emails sent to us. The rejection message is attached below,

....

<simon.lin@oscgc.com>: host mx.zohomail.com[74.201.154.201] said: 550 Mail

rejected by <Zoho Mail> for policy reasons. We generally do not accept

email from dynamic IP's as they are typically used to deliver

unauthenticated SMTP e-mail to an Internet mail server.

http://www.spamhaus.org maintains lists of dynamic and residential IP

addresses. If you are not an email/network admin please contact your

E-mail/Internet Service Provider for help. Email/network admins, please

contact <support@zohomail.com> for email delivery information and support

(in reply to DATA command)

.....

I added customer's email domain to the whitelist. Their email was still blocked with same error message. I just added our domain name to the Allowed Domain and don't know if it helps. Any suggestion to unblock the particular domain user?

Thanks.