I do not think that is true.

Once when this happened to me a couple of years ago, it was the opposite.

My e-mails were put by default by Microsoft as spam into the junk folder, without the customer knowing anything about this.

After I succeeded to notify him about this, he searched there the e-mails and marked them as "not spam", and then he received my following e-mails.

So initially the customer did nothing and was not aware that some of the e-mails sent to him are classified as spam, and he had to do active efforts to override this default action by Microsoft.

There was absolutely nothing suspicious about the e-mail messages classified as spam in their content, their only fault was not coming from one of the few major e-mail providers.