It appears that your forms are not using SMTP authentication before sending out mails, which is now mandatory for each and every form.
Kindly ensure that you are using that function by getting in touch with your developer in this regard.
phpmailer is one of the world's most popular mail client for PHP, it can be downloaded from
https://code.google.com/a/apache-extras.org/p/phpmailer/downloads/list
Example on how to set it up is at
https://code.google.com/a/apache-extras.org/p/phpmailer/wiki/BasicSmtp
If you still face any problem after enabling this please do feel free to get in touch with us.
