If you are using the WordPress plugin Contact-Form-7 on a Webserver that is not configured as a mail server you will get an error.

By default the error is: Failed to send your message. Please try later or contact administrator by other way.

The only solution I have found is to install, on my Ubuntu Server(s), a sendmail provider (i.e. sendmail, exim, postfix, etc…). After which the error goes away.