NDR error codes 550 5.7.703, 550 5.7.705, 550 5.7.708, and 550 5.7.750

550 5.7.703

The complete NDR error message is as follows:

This NDR is generated if a user in your organization sends mail to an email address or domain that's blocked by the tenant allow/block list. The list specifies external senders that are blocked from emailing users in your organization, and vice versa. If one recipient or domain in an email message is blocked, all other internal and external recipients in the message are also blocked.

How do I fix this?

Only an email admin in your organization can change the tenant allow/block list.

550 5.7.705

The complete NDR error message is as follows:

If your organization sends too much spam or bulk mail, Exchange Online blocks your outgoing mail and generates this NDR. Either of these common scenarios can cause spam or bulk mail to pass through your organization:

• A compromised on-premises server that routes mail to Exchange Online

• A compromised admin account that's used to create connectors

Also, if your organization renews an expired Exchange Online subscription, it might be blocked from sending email while the subscription renews (typically, for no longer than one day). This is a rare scenario because most organizations don't let their subscription expire.

How do I fix this?

Only an email admin in your organization can fix this issue.

If you're an email admin, see the following information about how to protect your organization from threats:

• Responding to a compromised email account

• Set up multifactor authentication for Microsoft 365

• Configure your Microsoft 365 tenant for increased security

• Microsoft Secure Score

After you address the underlying cause of the issue, contact Microsoft Support to have the block removed.

550 5.7.708

The complete NDR error message is as follows:

This NDR is generated if you send an email message from an IP address that has a low reputation. The issue is more likely to affect new customers, such as those who have a Microsoft 365 trial subscription.

How do I fix this?

Only an email admin in your organization can fix this issue.

If you're an email admin, and you receive this error during a trial subscription, contact Microsoft Support to request an IP address exception until you assign Exchange Online licenses.

550 5.7.750

The complete NDR error message is as follows:

This NDR is typically generated if a large volume of email messages are sent from a domain that isn't an accepted domain. Microsoft 365 only allows a certain amount of email messages to be sent from an unregistered (unaccepted) domain.

A less common cause is one or more incorrectly configured connectors.

How do I fix this?

Only an email admin in your organization can fix this issue.

If you're an email admin, follow these recommendations:

• Add and validate all domains in Microsoft 365 that you use to send email messages. This is the most common solution.

• Use a certificate-based outbound connector. The certificate's domain must be an accepted and validated domain in Microsoft 365 or Office 365. For more information, see Configure mail flow using connectors in Exchange Online.

• Look for unusual connectors and compromised accounts. Attackers sometimes create new inbound connectors in organizations to send spam. For more information, see Validate connectors in Exchange Online and Respond to a compromised email account.

After you address the underlying cause of the issue, contact Microsoft Support to have the block removed.