I recommend asking at https://wordpress.org/support/plugin/wp-mail-smtp/ first and then maybe at https://wordpress.org/support/plugin/contact-form-7/ so the plugin’s developers and support community can help you with this.
Kind regards!
You’ve apparently enabled reCAPTCHA on the form, by supplying the API keys to the CF7 settings. The scoring returned by reCAPTCHA can cause mysterious failure errors. The easiest way to see how reCAPTCHA scores a submittal is to install and activate CF7’s companion plugin “Flamingo”. It logs form submittals and shows the reCAPTCHA scoring.
Won’t solve the problem, but will indicate whether reCAPTCHA is the cause or not.