I have the same error since the last update (three days ago or so) as mentioned in some other threads (but as mentioned in those by a moderator I will post a new topic even if it seems to be the exact same issue).
Whenever the submit button is clicked I get the error "Failed to send your message. Please try later or contact the administrator by another method."
Until the update everything worked just fine for many month and I changed nothing in my forms. So it can't be wrong configuration. The update installed and there was this error. With all of my forms.
As I understand this message says the submits are marked as spam.
The error comes whatever I do. I set up a new form with just standard setting, removed the field for mail, added an removed captcha - the error remains the same.