The error is caused when your server or hosting has an issue with the PHP Mail function. You need to contact your hosting provider to resolve the issue.
Here are some hints what may cause the error.
- Sender email address is not locally hosted mailbox or using a different domain
- Sendmail or postfix on your server/hosting do not working properly (contact your hosting provider, or you can try to use local SMTP available in your server/hosting)
- Notification emails are disabled from the Admin CP
- Sender's name and/or email address is invalid
- Recipient's name and/or email address is invalid
- There's a special character in the subject email and/or in the sender/receiver information
- You already sent too many emails (and restricted by server)
- Invalid attached file (e.g. to large, file not allowed by server)
- etc