Fix Microsoft 365 Connection Limits When Sending Newsletters with Celery

Configure your Celery worker concurrency (e.g., using the `--concurrency` flag) to match the maximum allowed simultaneous connections for your Microsoft 365 mailbox tier (typically 3 concurrent connections).

Apply the `rate_limit` attribute to your Celery email sending task (e.g., `@app.task(rate_limit='30/m')`) to prevent the queue from dispatching messages faster than Exchange Online can accept them.

Wrap your email dispatch code in a try-except block that catches SMTP or API rate limit exceptions (like HTTP 429), and use Celery's `self.retry(exc=e, countdown=60)` method to securely back off and try again.

Upload your newsletter attachments (such as PDF reports or promotional images) to a secure cloud storage solution like OneDrive, SharePoint, or an external CDN.

Update your HTML newsletter templates to include direct, secure hyperlinks to the hosted files rather than embedding them directly into the MIME payload.

Run a small test batch through Celery to verify that the reduced payload size resolves timeout issues and speeds up the overall processing queue.

Log in to the Microsoft Entra ID admin center, navigate to your registered app, and confirm that `Mail.Send` (Application or Delegated, depending on your architecture) is granted and admin-consented.

In the Authentication tab of your app registration, ensure the configured redirect URIs precisely match those used by your OAuth client during the initial token exchange.

Update your Python script to automatically detect token expiration before dispatching an email, securely fetching a new access token using the stored refresh token to prevent mid-batch authentication failures.