sergio_alifano on "Contact Form 7 working only on localhost"

I have no idea why but it used to work on production.
Now it doesn't work anymore but if I make test on localhost I receive the emails

I have the following message:
"Failed to send your message. Please try later or contact the administrator by another method."

Any idea?
Thanks

https://wordpress.org/plugins/contact-form-7/

FAQ > After submitting the form, I get an error message stating “Failed to send your message”. What is wrong?

I did already read this article but it didn't help...
I also tried to use others plugin but they all work only on localhost. It might be some configuration issue with the hosting service I guess.