Spam filters can respond to certain words if they are present in the header or body of the email. I try to avoid using the following words (they were found by experience, and I do not guarantee that this is the correct list)
You need to set up dkim, spf, dmarc. The user must also activate the subscription. And when he signs, display a message: "If you did not find our letter, check your spam folder. If it is there, check the "not spam" box next to the letter."