I had a Request from a Reader who has claimed that his Message sent with the Contact form was sent back to his Mail Account because the Contact Form SEEMS not to work and he said that he had received the Same message.
Then i was searching for the Mail in my Account and what a Miracle- the Mail was sent properly over the Contact Form.
I would test it with more then one Email account and if it doesn't works i'd change the Contact Form in the Manner that you have to Use a Username conclusively. If that doesn't work: Deleted the Plugin and reinstall it and make a new Setup maybe you have changed something without intention?
You know, with WordPress and all this Plugins it's pretty similar like with Windows and all this masses of Programs outthere: One Setup works another doesn't and it's impossible to test all possible Combinations. WordPress, like Windows is a extremely complex Software with Thousands of Files and it always can happen that a Certain Combination of Programs and Versions, here WordPress Versions and Plugins simply do not work together, because all this Settings in all this Files aka Pluginss often influences each other and can Knockout each Other too.
In this Case you just can wait until the Author of the Plugin provides an Update or you just search for another Plugin with the Same Functionality. that does help in 99.9275 % of the Cases.
Hope that one of these Options will work for you.