Most probably it's due to a spam filter. Check your spam filter settings and spam/bulk mail folder.
Check also the mailing method allows by your hosting service, maybe you need to put a different "from" address or a different "smtp" server address
Please, be sure to upload the "DEX_Form2MailPHP" folder and files inside. After that, if you are still getting the error, please check the path to the ".php" file in the form's action attribute.
The "DEX_Form2MailPHP" folder is generated (automatically) in your local web site when you insert the extension in your form.
If you want to prevent automatic submissions you can add a security verification image (captcha) into your forms. We have a captcha extension here.
You can download the Adobe Extension Manager from the following page:
On Linux/Unix based webservers, FTP programs ("clients") allows setting permissions for files and directories on your remote host. This function is often called chmod or set permissions in the program menu.
Writing permissions is similar to "777" permissions or "rwx-rwx-rwx" permissions. You may see some of these representation in your FTP program.
On Windows based webservers the permissions should be set through the webhosting control panel.