This error occurs when a service (typically SMTP) has received too many authentication failures for a specific username and will prevent any further authentication attempts for that username for a period of time.
This typically will stop a Nylas account and require that the user wait some time before re-authenticating with Nylas.
Error received when sending emails.
If you receive this error back when trying to send an email AND the Nylas account's Synchronization Status is not Invalid or Stopped, this means that only the SMTP service has blocked authentications and Nylas is still connected to the mailbox via another service like IMAP.
If this is the case, please retry the send in a few minutes. Please reach out to the user's email administrator if you are still unable to send. Re-authenticating with Nylas may not address this since we still have good credentials if the Synchronization Status is not Invalid or Stopped.