Block Microsoft Outlook Emoji Reactions in Zammad

[YetAnotherGerrit]

As an agent, I want Zammad to automatically block Microsoft Outlook’s emoji reactions (sent via the x-ms-reactions header) for all outgoing emails. This ensures that agents do not receive useless or unreadable content in their emails, improving the overall email experience in Zammad.

Acceptance Criteria

• Zammad automatically adds the header x-ms-reactions: disallow to all outgoing emails.
• Incoming emails with emoji reactions are not ignored but are expected to stop arriving once the header is sent.
• No configuration or override options are provided; the feature is global and mandatory.
• Compatibility with major email clients needs to be tested, and any issues are documented.

[YetAnotherGerrit]

If we ever decide to allow reactions to articles in Zammad, this can be revised.

[fliebe92]

For reference: https://techcommunity.microsoft.com/blog/outlook/reactions-in-outlook-public-usability-update-september-2023/3928103