452 Mailbox delivery restricted by policy error
We have been testing Zoho desk for about a week now and have been forwarding emails in via an Exchange Online Mail flow rule without issue until yesterday. Suddenly yesterday morning we started getting the vast majority of the emails stuck in Pending and in the message trace is the event
Reason: [{LED=452 Mailbox delivery restricted by policy error <support@*******.zohodesk.com>};{MSG=};{FQDN=zm-apps.zoho.com};{IP=136.143.182.216};{LRT=3/11/2026 5:21:25 PM}]. OutboundProxyTargetIP: 136.143.182.216. OutboundProxyTargetHostName: zm-apps.zoho.com
This appears to be a recipient issue, which so Zoho Support seems to agree with by the response I got back,
The error “452 Mailbox delivery restricted by policy” indicates that the recipient’s mail server has temporarily blocked the email due to its internal security or delivery policies. This can occur if the server detects a high volume of emails being sent within a short period, enforces strict filtering rules, or temporarily restricts messages originating from external mail servers.
However, then the solution provided makes no sense to me as they then state:
Since this restriction is enforced by your mail server, we recommend attempting to resend the email after some time. If the issue continues, please contact your IT or mail administrator may need to review the mail server policies and whitelist the sending server or domain to ensure emails are accepted.
I'm a little confused on what I'm missing here. I was under the impression whitelists were for inbound messages not outbound. If there is an outbound whitelist can someone point me in the right direction, because I have never had to whitelist for an outbound email before.
Anyone else ever seen this issue before?