Why is the raw html markup of a Laravel email output?

Question

Laravel

I set up mail distribution in Laravel and the letters were formed normally. Then at some point (although I did not change the mailing method), the letters began to be sent in the form of raw html markup. What could fly? Here is a piece of code:

Mail::to($event->reply->post->owner->email)->send(
            new ReplyCreatedMail($event->reply)
        );