message failed to deliver

Hello,

I use the same WordPressversion and CF7 Version on my domain and a separate subdomain. On my regular domain CF7 works fine and without any problems. On my subdomain, CF7 always shows the PopUp

“Failed to send your message. Please try again later or contact administrator by other way.”

this is the message #4, the one for spam.

It doesn’t matter which Email I type in or what my message says, I always recieve this errormessage.

I do not use Akismet because it is not perfectly legal in Germany, as I heard.

I hope you can help me…

Wp-Version: 4.5.2
CF7 Version: 4.4.2

https://www.ads-software.com/plugins/contact-form-7/