Microsoft® 365™ users are able to send emails to other users on the Microsoft 365 domain, but when sending to a non-Microsoft 365 account, they receive a Non Delivery Report (NDR) from Microsoft 365. An example is shown below:

"When Microsoft 365 tried to send your message to the next server outside Microsoft 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.

More Info for Email Admins
Status code: 550 5.7.368

The sender's message was routed to an email server outside Microsoft 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 <Microsoft _365_domain>..."

This issue can occur if connectors have been enabled in the Microsoft 365 domain's Exchange Online, 'mail flow' settings and those connectors are either no longer active or are faulty. The connectors can be to a trusted partner organisation or to a third party product (mail security, spam filter etc.) not affiliated to either RM or Microsoft 365.

1. Log on to https://portal.office.com/adminportal/home#/homepage as a Microsoft 365 global administrator.
2. In the left-hand pane, click 'Admin centers', Exchange. The 'Exchange admin center' window opens.
3. In the left-hand pane, click 'mail flow'.
4. On the top right-hand side, click 'connectors'.
5. Select any connectors listed in turn and review the information shown in the details window on the right-hand side.
6. If any connectors for third party companies are present, contact them for further advice they may have on potential outages, DNS changes, firewall issues etc. affecting your service.