Using the Send a Message endpoint to send an email may result in a '503' error. See the example below.
503: Unknown connection error - provider's SMTP server isn't reachable
This error occurs when Nylas isn't able to connect to the provider's SMTP server to delivery the email. As a best practice, have a retry mechanism with exponential backoff e.g. in seconds retry_intervals = [30, 60, 120, 300, 600] , once the /send attempt >len(retry_interval) . If the issue persists, please contact your e-mail administrator.
We have observed this behaviour if the developers application sends multiple emails simultaneously. Ensure you follow Improving Email deliverability