Use the x-ms-reactions header to disallow reactions in Outlook

Microsoft Outlook reactions provide a lightweight method for recipients to acknowledge or respond to email messages without sending a traditional reply. However, in some scenarios, such as sensitive communications, formal announcements, or compliance‑related messages, senders or organizations might want to prevent reactions entirely.

The x-ms-reactions message header provides a server‑enforced mechanism to disallow reactions on individual email messages across Outlook clients.

Summary

This article explains how to use the x-ms-reactions message header to prevent recipients from using reactions in Microsoft Outlook. Learn about server-side and client-side enforcement mechanisms, and how to implement reaction restrictions through both Outlook clients and Exchange Online mail flow rules.

About the x-ms-reactions header

Exchange Online and Outlook clients recognize x-ms-reactions as an SMTP message header that indicates whether reactions are allowed for a specific email message.

The supported value for x-ms-reactions is:

x-ms-reactions: disallow

If this header is present on a message, reactions are explicitly disallowed. This behavior is enforced regardless of how the message is sent or which Outlook client is used to view it.

You can apply the header by using an Outlook client or administratively through Exchange Online.

Server-side enforcement: Exchange Online blocks reaction attempts

If Exchange Online detects x-ms-reactions: disallow on a message:

• Exchange Online blocks all reaction attempts at the service layer.
• Reaction submissions fail even if a client still displays reaction controls.
• Enforcement is consistent across Outlook platforms and versions.

This server‑side behavior makes sure that reactions are reliably suppressed even during phased client rollouts or in mixed client environments. Exchange Online is the authoritative enforcement point for disallowed reactions.

Important

Server‑side blocking doesn't depend on client UI support. Even if an Outlook client doesn't disable or gray out reaction controls in the message UI, Exchange Online prevents reactions from being recorded or delivered.

Client-side enforcement: Outlook blocks reaction attempts

Outlook clients support reactions in two related but distinct manners:

• When composing. Senders can disallow reactions from recipients when writing an email message. Disallowing reactions during composition adds the x-ms-reactions: disallow header to the outbound email message.
• When reading. Outlook disables or grays out reaction controls when a recipient views an email message that includes the x-ms-reactions: disallow header. In this case, the header might be added by an Outlook client or by a mail flow rule in Exchange Online.

Combined client support matrix

Outlook client	Compose UI: Disallow reactions on send	Read UI: Honor x-ms-reactions: disallow
Outlook on the web	Available	Supported
New Outlook for Windows	Available	Supported
Outlook for Mac	Available	Supported
Outlook for iOS	Available	Supported
Outlook for Android	Available	Supported
Classic Outlook for Windows (desktop)	Not available	Not supported

Receiving a message that includes the x-ms-reactions header

When a recipient receives an email message that includes x-ms-reactions: disallow:

• Supported clients disable or gray out reaction controls in the UI.
• Clients that don't use this UI behavior might still display reaction controls.
• Exchange Online blocks all reaction attempts regardless of the behavior of the UI.

Use mail flow rules to add x-ms-reactions and disallow reactions in Exchange Online

As an Exchange administrator, you can add x-ms-reactions: disallow to email message header fields by using Exchange Online mail flow rules. These rules show how you can automatically apply the header to messages for which reactions aren't appropriate.

Common inbound scenarios include:

• Messages that you receive from external senders
• Messages that are addressed to large distribution lists or shared mailboxes
• Messages that match specific compliance or sensitivity criteria

When you apply the header through a mail flow rule:

• Exchange Online inserts the header on the server side.
• No client support or user action is required.
• Exchange Online consistently enforces reaction blocking.

By using mail flow rules, you make sure that reactions are disallowed on inbound email messages. This restriction includes email messages that users view in clients that don't support reaction UI suppression, such as Classic Outlook for Windows. For more information, see Mail flow rule actions in Exchange Online.