How do I fix a 504 / 503 error when sending an email using the API?


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 reach the provider's SMTP server to send the email. As a best practice, have a retry mechanism with exponential backoff e.g. try sending once, then try again ~1 sec later, then back off exponentially for ~5 attempts with a maximum backoff interval of 1 minute. 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


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