mails are sent and received, but ‘Mail Delivery failes’

I sent a mail from my gmail address to my iCloud address, just to test HappyForms.
On my iCloud address (receiver) I received the message ‘I received your mail’, so that’s a confirmation that my mail has been received.
On my gmail address (sender) I received the filled in HappyForms form. So that’s OK too, but 15 – 20′ later I received on my gmail address (sender) a mail: Mail Delivery System: This message was created automatically by mail delivery software.

A message that you sent could not be delivered to one or more of its
recipients. This is a permanent error. The following address(es) failed:…

What happens and what can I do?

Thanks for all reply