Microsoft don't include the body of previous messages in a Thread

Why don't my email replies show the full conversation thread in Microsoft Outlook recipients' inboxes?

Microsoft has changed its email threading behavior and no longer automatically includes previous message content in threaded email replies. This affects how recipients see conversations in their Outlook inboxes.

When you send a reply using the Nylas API with the correct reply_to_message_id subject formatting ("RE: Original Subject"), the messages are technically threaded correctly. However, Microsoft Outlook recipients will see each reply as a separate message without the full conversation context.

To provide the full conversation experience, manually include the previous message content in your email body.

Important Considerations:

• This is a Microsoft provider-side limitation, not a Nylas API issue
• The threading headers are still set correctly by Nylas
• You'll need to manage conversation history in your application
• This behavior affects all email APIs working with Microsoft accounts, not just Nylas

June 20, 2025 22:07