The message looks like spam

If you have sent a message to a Yandex address and received a non-delivery report that states the following: “Message rejected under suspicion of SPAM”, this means that the contents of your message was recognized by Yandex.Mail as spam.

If only spam is coming from your mailbox, the Yandex mail server will block receipt of messages from it for 24 hours. In this case, the non-delivery report contains the line “Client host [<IP_host>] blocked using spamsource.mail.yandex.net; see...” or “Blocked by spam statistics — see...”.

Note. If you are sending legitimate messages to your website users or subscribers, please read Yandex requirements for honest mailing lists.