Update 04/13/2021 14:39 PM 7 days

Work is still ongoing with outbound-auth and outbound-xdgrid.

We were also made aware of bounces from M365 clients that use our smarthosts. The issue that caused that problem has been resolved, if your message bounced (if you received an error similar to below) please resend it.

Your message couldn't be delivered to the recipients shown below.
Office 365
Multiple recipients
Sender
Action Required

Sender not authorized for relay
Couldn't deliver the message to the following recipients:

How to Fix It
When Office 365 tried to send your message to the next server outside Office 365, that server reported an error that it couldn't relay your message. It's likely that the email server isn't correctly set up to receive and relay messages from your organization.
To fix this issue, forward this non-delivery report (NDR) to your email admin.
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More Info for Email Admins
Status code: 550 5.7.367
The sender's message was routed to an email server outside Office 365 that returned an error that it can't relay the message. It's likely that the server isn't set up correctly to receive and relay messages sent from yourdomain.com or from youruser.

Update 04/13/2021 13:28 PM 7 days

DNS problems have been addressed and outbound mail flow has returned to normal across all data centers.

We are still addressing an issue with outbound-auth (used by IoT) and outbound-grid (used for Yahoo/BT/freemail system retries when they sporradicaly have issues) where performance is still degregated. 

Update 04/13/2021 11:22 AM 7 days

We are currently experiencing reduced performance across our ExchangeDefender outbound networks. The issue is DNS related and we're making adjustments to network resources to help improve performance.