Sendmail is a mail transfer client which is used by many providers

  1. Make sure the path “/usr/sbin/sendmail” is correct
  2. Make sure you have permission to send with sendmail (ask the provider for help). Shared hosts often disabled this function.
  3. Try a different sending method at Settings => Newsletter => Delivery

Sendmail isn’t designed for bulk mailings in the first place. However, you can still use it for this purpose. Some providers have limits so you may have successfully sent emails via Sendmail and it suddenly throws that error.

If you like to bypass this error it’s recommended to use a dedicated email provider.¬†Use one of these free plugins on the WordPress site.