There can be many reasons, here are some reasons why your emails end up in the Spam folder or may end up:
Sometimes it happens that a letter can end up in spam, only because the text is very similar to one that is already in spam;
Do not write a lot of recipient addresses in the letter, otherwise, not immediately, but soon it will be sent to spam;
If the user you are writing to has already sent your letter to spam, the next one may go there automatically;
If the letter contains no text, but only a picture, and not a small one at that, then the letter will most likely also end up in spam.
Do not forget that each mail server has its own individual settings.