Outlook Error Codes

How to Fix Outlook Error 503 5.5.1 When Sending BCC Emails

Natalie Taylor Sep 28, 2026 868 views

Question details

The user is encountering an Outlook Error 503 5.5.1 when attempting to send emails to multiple recipients using the BCC field.

How to Fix Outlook Error 503 5.5.1 When Sending BCC Emails

Product: Microsoft Outlook
Device & OS: not provided
Scenario: Sending an email to approximately 45 or more recipients simultaneously via BCC through Gmail or Exchange servers.
Observed behavior: The email fails to send, and the system returns a nondelivery report (NDR) indicating error code 503 5.5.1.

Before you start

Ensure you have administrator access to your Microsoft 365 or Exchange environment, as diagnosing this specific error often requires running a message trace.

Solution 1Recommended

Analyze the Nondelivery Report and Run a Message Trace

Identify the root cause by reviewing the exact bounce message and tracing the failed email in the Exchange admin center.

Error 503 5.5.1 typically occurs due to server authentication issues, rate limits, or spam filter triggers when sending to a large BCC list.

Open the nondelivery report

Locate and open the bounce-back email (NDR) you received in your Outlook inbox.

Review message headers

Examine the message header for specific failure codes that indicate whether the block originated from Exchange or your external provider (like Gmail).

Access Exchange Admin Center

If you are a Microsoft 365 Administrator, log into the Microsoft 365 admin portal and navigate to the Exchange admin center.

Navigate to Message Trace

Click on 'Mail flow' in the left-hand navigation pane and select 'Message trace'.

Run the trace

Input the sender's email address and the specific time frame of the failed message, then run the trace to find the exact reason for the block.

Analyze the Nondelivery Report and Run a Message Trace

Privacy Warning: Always redact personal email addresses and sensitive data before sharing diagnostic headers with external support teams or forums.

Solution 2

Verify SMTP Authentication and Sending Limits

Ensure your email client is properly authenticated and you are not exceeding the recipient limits set by your email provider.

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Frequently Asked Questions

What does Outlook Error 503 5.5.1 mean?

Error 503 5.5.1 typically indicates an authentication issue with the outgoing SMTP server, or it means the sender has triggered spam filters by emailing too many BCC recipients at once.

How many BCC recipients can I send to in Outlook?

The limit depends on your email provider rather than the Outlook application itself. For example, Microsoft 365 limits messages to 500 recipients at a time, while third-party clients using Gmail may be restricted to fewer recipients per day.

Why do emails fail only when using BCC?

Sending to a large number of undisclosed recipients via BCC is a common tactic used by spammers. Email providers like Exchange and Gmail often flag these messages and block them to protect their server reputation.

How do I run a message trace in Microsoft 365?

As a Microsoft 365 administrator, navigate to the Exchange admin center, go to the 'Mail flow' menu, and select 'Message trace'. Enter the sender and recipient details to track the email's delivery status and error logs.

Microsoft Office Error Codes