Email not received. The sender received NDR with error code: 550;5.7.703

Products

• Email Security.cloud

Issue/Introduction

Symptoms:

1. The email was sent but not delivered

2. The sender receives NDR.

3. NDR contains the following or similar communicate:

550; 5.7.703; Your message can't be delivered because messages to XXX, YYY are blocked by your organization using Tenant Allow Block List

Environment

Email security cloud

Cause

The issue is created by local policy restrictions of the recipient. It is not caused by Symantec. Therefore, NDR is only a result, not the reason.

Resolution

An admin in your organization used the Tenant Allow/Block List in Microsoft Defender for Office 365 to block an email address or domain from sending email into your organization. This block entry also prevents users in your organization from sending email to those same email addresses. The entire message is blocked for all internal and external recipients of the message (XXX and YYY in the example), even if only one of the recipients is blocked using the Tenant Allow/Block List.

Please refer to Microsoft article about this issue to get more details.

As resolution, the sender must revise his Microsoft Defender settings. Also, you can raise a support case to Microsoft. Additionally, please make sure that recipient whitelisted Symantec IP range in his environment.

Additional Information

22-Dec-2025: created an article