When using Campaigns to send emails, an “error” status can occur in case your emails cannot be sent due to an error.
There are two possible Error messages you will see when you try to send emails that have an Error sending status:
1. The server responded with status 400
This error can occur when you have a custom setup on your end. For instance, using Exchange Servers with a hybrid/custom deployment.
This seems to be a known issue with Microsoft that other apps are facing that hasn't been resolved. Due to that, you could look into the setup that you currently have for your email account
This issue can occur when there is a custom Outlook setup in play. This means you will need to look into your setup and/or contact Microsoft to see if they can resolve this issue on their end.
2. The server responded with status 429
This happens when too many requests have been sent to the email provider's API. This means that you have probably sent many emails too quickly.
This can also happen if you are using the Outlook/Gmail API from another platform and that causes you to go over the number of requests that Outlook/Gmail allows.