@clarkwinkelmann @VerbMaestro @phenomlab Thank you for your ideas but I just solved the problem and this is important. The problem was -well, mostly because of me- on Flarum's side.
There's a Forum Title selection in the Basics section. What I entered there was too long. I did it because of SEO purposes. I made it too long.
What happens is this; when mail gets send, forum title is a part of the mail title. "[FORUM_TITLE] ...." like this.
My forum title was too long. It was probably longer than what was acceptable as an email title. My mails were bouncing back because of this.
Please implement character limit in the forum title. So others won't have these issues.