Thank you for your advice, concerning Aimy CaptchaLess... I have already tried this, because in earlier days I had experienced, that the extensions by Aimy are of very good quality and trustworthy.
Sorry to say, that Aimy CaptchaLess had no effect on AcyMailing forms. I just have read the documentation of Aimy very thoroughly and I have found this info:
Aimy Captcha-Less Form Guard is then used as a default captcha for your Joomla! website and will be used whenever an extension requests the default captcha to be displayed.
But I still do not have Acymailing as a licensed version and thus Acymailing will not show up a captcha, which could be replaced by Aimy.
As I will not use any of the Pro features of Acymailing, the feature of having a captcha is the only thing for me, that should be worth nearly 30 Euro and the it is for one website only.
I will try to insert hidden fields in the default form of Acymailing. But as additional fields in Acymailing are part of the licensed version, I suppose, that it would not be easy, to insert those hidden fields by simply changing some php defined forms.