The most common cause of this error in either PingPlotter or MultiPing is an invalid SMTP server specified. The error means that the name entered couldn't be resolved to an IP address.

In MultiPing, this is in the "Edit" -> "Options", Email tab. In PingPlotter, this is on the Edit->E-mail... setup menu.

Make sure you're using the right SMTP Server name or IP address there. If you're not sure what to use, check the settings in your email client, or check with your ISP.

It's also possible that the network may have been down right when you did this test. If this was the case during a network "event", then the email would have been retried until it went through.

Because this error is related to resolving a name to an IP address, one way to eliminate it is to enter an IP address as the SMTP server.