A loop was detected in sent messages.
If you sent an email from your Yandex mailbox and received the non-delivery report “Loop detected! Message rejected”, “Mail for <recipient's email address loops back to itself”, “Our servers have detected a mail loop”, this means that forwarded messages were looped due to improperly configured forwarding rules or mail importer.
Check the forwarding rules and importers on all mailboxes that you are using. If you are sure the problem is not on your end, contact the recipient by an alternate means of communication to report the problem.