Security Policy Errors

Fix Exchange Mail Flow Rule Reject Message Text Invalid Error

Rana Garcia Oct 19, 2026 868 views

Question details

The user needs to resolve a validation error preventing them from saving a custom rejection message in a Microsoft 365 Exchange mail flow rule.

How to Fix Invalid RejectMessageReasonText in Exchange Mail Flow Rules

Product: Microsoft 365 Exchange
Device & OS: not provided
Scenario: Creating an Exchange mail flow rule to reject incoming messages and provide the sender with a customized text explanation.
Observed behavior: The system throws an error stating that the RejectMessageReasonText or delivery status notification text is invalid, specifically occurring when the custom text contains line breaks.

Before you start

Ensure you have the necessary administrator permissions in the Exchange admin center to create or modify mail flow rules.

Solution 1Recommended

Remove Line Breaks from the Rejection Message

Format your custom rejection text as a single, continuous paragraph without any line breaks to bypass the validation error.

Exchange mail flow rules often struggle to process custom notification messages that contain line breaks or paragraph returns. Condensing your message into a single block of text is the most reliable workaround.

Copy the message text

Highlight and copy your intended custom rejection message.

Paste into a word processor

Open a word processing application like WPS Writer or Microsoft Word and paste the copied text into a blank document.

Delete all line breaks

Manually delete all paragraph returns and line breaks so that your message forms one continuous line or paragraph.

Copy the formatted text

Select the newly condensed, single-paragraph text and copy it to your clipboard.

Update the mail flow rule

Return to the Exchange admin center, paste the formatted text into the RejectMessageReasonText field of your mail flow rule, and save the rule.

Remove Line Breaks from the Rejection Message

Formatting Limitations: Although the text will display to the sender as one long paragraph without line spacing, this adjustment guarantees the rule will be saved and executed successfully.

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Frequently Asked Questions

Why does Exchange reject mail flow rule text with line breaks?

Exchange mail flow rules have strict syntax validation for custom rejection messages (RejectMessageReasonText). Line breaks and certain special characters can interfere with the underlying formatting code, triggering an invalid text error.

Is there a character limit for the Exchange rejection message?

Yes, Exchange imposes limits on the length of rejection messages. Always ensure your custom explanation is concise and falls within the character limit specified by your Microsoft 365 environment, which is typically a few hundred characters for basic transport rules.

Can I use HTML to format the rejection message instead?

By default, the RejectMessageReasonText field in standard mail flow rules only accepts plain text. Using HTML tags will not render as formatted text to the recipient and may also trigger further validation errors upon saving.

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