Email issues

Hello Gilles,

I understand that two accounts cannot send emails, although both accounts are enabled and users can sign in successfully. The outgoing messages are returning NDR errors 550 5.7.703, 550 5.7.705, 550 5.7.708, and 550 5.7.750.

Please ask your Microsoft 365 email administrator to check the following, based on the error returned:

1/ Check the Tenant Allow/Block List

1. Sign in to the Microsoft Defender portal.
2. Go to Email & collaboration > Policies & rules > Threat policies > Tenant Allow/Block List.
3. Check whether the affected recipient addresses or domains appear in the block list.
4. If a legitimate recipient is blocked, review and update the entry. Reference: 550 5.7.703

2/ Check outbound email activity

This error means the organisation has exceeded its outbound email threshold.

Ask your administrator to check for compromised on-premises servers, compromised administrator accounts, or unauthorised mail flow connectors. Also, check whether an expired Exchange Online subscription was recently renewed. Reference : 550 5.7.705

3/ Check the sending IP address

This error means Exchange Online is rejecting messages from an IP address with a low reputation. If your organisation is using a Microsoft 365 trial subscription, the administrator should contact Microsoft Support to request an IP address exception until Exchange Online licenses are assigned. Reference: 550 5.7.708

4/ Check accepted domains and connectors

1. Open the Exchange admin center.
2. Go to Mail flow > Accepted domains and confirm that all domains used to send emails have been added and validated.
3. Go to Mail flow > Connectors and review the configured connectors.
4. If a certificate-based outbound connector is used, confirm that its certificate uses an accepted and validated domain.
5. Check for unfamiliar connectors or compromised accounts. Reference: 550 5.7.750

For more information, see Microsoft's troubleshooting guide for these NDR errors.

Kind regards,