Sometimes the email client is configured in a way that you have to specify an admin email address of the same domain as the one of your site to make the forms work. It has something to do with " Bad SPF records"...
If your site is www.mydomain.com, then set your admin email address to admin@mydomain.com (and then if you like, set up redirections afterwards, etc)
This worked for me for contact form. Maybe other forms are handled similarly... Anyway, I don't know if it might help, but try it if you want, it's a simple change (in "Site settings" link at the top of your site).
