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

  1. Make sure the the path “/usr/sbin/sendmail” is correct
  2. Make sure you have the permission to send with sendmail (ask 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 successful sent mails via Sendmail and it suddenly throws that error.

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