Apparently, even optimally set up email servers with IPs with a good history are blocked by Google and other mail services without a bounce response, or with one that is days late, supposedly due to a low reputation. (e.g. https://www.heise.de/news/Google-stufte-ct-de-als-Spamschleuder-ein-9241222.html)
Does anyone have experience with this, solutions or want to exchange ideas on how to solve the problem of non-delivery (e.g. in professional circles where you also have to send to a gmail account)? Contact for a meet-up: 37c3anony@posteo.de