Email Delays to Yahoo Domains

We have been investigating delays when customers are sending emails to Yahoo domains such as yahoo.com, yahoo.com.au, yahoo.com.uk, etc. The top error message received has been “421 4.7.0 [TSS04] Messages from 67.219.246.1 temporarily deferred due to unexpected volume or user complaints - 4.16.55.1; see https://postmaster.yahooinc.com/error-codes”.

We have concluded that the large volume of 421 errors are due to EchoSpoofing email runs originating from attackers abusing Microsoft 365 to impersonate legitimate customers and relay outbound email through our service. This has caused Yahoo to throttle messages sent from some of our servers.

Over the past months we have been taking steps to block these emails including adding Data Protection rules, and proactively enabling the new Anti-Echo Spoofing Protection feature for customers being targeted by the attackers. This means that the vast majority of our customers are now protected and we are about to take the final step of enabling Anti-Echo Spoofing Protection for those few who have not yet turned it on themselves. If you already have an Anti-EchoSpoofing Data-Protection rule in block or quarantine, or if you have enabled Anti-Echo Spoofing protection in block mode then you are already protected and no action will be taken.

If you experience any issues sending outbound email please check your list of authorised Organization IDs and make sure that you have approved all legitimate sources. See Authorizing Microsoft 365 tenants for more details.

We continue to investigate the cause of intermittent delays to Yahoo domains. We will provide updates as new information becomes available.

We continue to investigate the cause of intermittent delays to Yahoo domains. We will provide updates as new information becomes available.

We are investigating additional reports of intermittent delays to Yahoo domains. We will provide updates as new information becomes available.

We are no longer experiencing delays delivering to Yahoo domains. We will now monitor the service for stability and provide updates as new information becomes available.

We continue to work on a solution. We will provide updates as new information becomes available.

We continue to work on a solution. We will provide updates as new information becomes available.

We have identified the cause of the issue and our engineers are continuing to work on a solution. We will provide updates as new information becomes available.

We continue to investigate the issue. We will provide updates as new information becomes available.

We are currently experiencing issues delivering emails to Yahoo domains. The result is that emails are queued for delivery.

Please note that there is no impact to protection services, and all other aspects of our services remain functional. We are investigating this issue on our side and are engaging Yahoo as appropriate.

Broadcom will continue to deliver emails based on standard and custom retry email schedules configured by our customers. We will provide an update as soon as more information becomes available.