Some web hosts do not allow PHP to send email unless the envelope sender email address is on the same web domain as your web site. And they require it to be a real address on that domain that is configured in the Web Host Web Panel, or mail will NOT SEND! (They do this to help prevent spam.)
If your contact form does not send any email., then set the “Custom E-mail From:” setting to a real email address on the SAME domain as your web site, then test the form.
The theme you are using could be causing it…
Do this as a test:
In Admin, click on Appearance, Themes.
Temporarily activate your theme to a WP default one…
“WordPress Default” theme(default for WP 2.0+), or
“Twenty Ten” (default for WP 3.0+), or
“Twenty Eleven” (default for WP 3.2+) or
“Twenty Twelve” (default for WP 3.5+)
It does not cause any harm to your WP to temporarily change the theme, test if the errors went away, and then and change the theme back.
Does it work properly with a default WP theme?
If it does, then the theme you are using is the cause. Use a different theme or report the errors to the theme developer.
Maybe another plugin is conflicting.
Look on the Admin – Plugins – menu.
Temporarily Disable (not uninstall) all your other plugins.
Did the problem you were having go away? If yes, Activate each of the plugins, one by one, to determine which one conflicts. Test if the problem goes away after activating each plugin one at a time. This way you can determine which plugin was causing the problem. Once you determine a plugin is conflicting, check the plugin version to be sure it is compatible with your version of WP. Sometimes plugins are no longer updated, then new WP versions come out and the plugin will generate errors or no longer function as intended. Report any problems you find to the plugin author.
