How did the mailbox get into the spam list?

Question

Fight against spam

I almost never used a mailbox on Yandex, I never indicated it during registration, I did not give it to anyone. I used it several times to send a letter to my main mailbox (on mail.ru), for example, so that when I'm at work, send a link and not forget about it (I'll come home, I'll sort out the mail and see it). Did this three times. Yesterday was the third.
And today came spam. How?

Answer 1

There was something about employment, right? Curious. Today I also received two letters specifically to the box on @yandex.ru, one was not recognized as spam and even got into the "inbox". Previously, the box was tied with bonds of full forwarding of correspondence with mail.ru, which were later broken.
With such scant information about the two cases, however, it can be assumed that Mail.ru, not Yandex, is to blame. Nevertheless, I was quite surprised that one email not identified as spam received the "Important" flag.

Answer 2

An email address before @ can be picked up by simple brute force.

Answer 3

They spam not only according to email lists, but also according to the dictionary <common word>@domain.tld