This just started happening to me too! All of the fields are correct, but I’m getting the same error message as eighty6.
Though the plugin successfully sends millions of emails every day, there are a host of issues that can stop or delay emails on both the sending and receiving ends. It depends entirely on your local Server & WordPress configuration. You will need to investigate this issue for your particular local configuration.
See Contact Form 7 Email Issues. There is a link at the bottom of the page to a comprehensive and detailed article which covers many issues. Most people that work through this information appear to be able to resolve their email sending & receiving issues.
This started happening to me as well. It’s all working great but now i cant use it. I have tried every possible solution but to no avail. I might try to kill someone LOL.