Some Russian email providers seem to have started marking messages with encrypted contents as spam by default. The mechanism and criteria are rather unclear (whether they check if the account is verified with a phone number / has Gosuslugi confirmation, or even go as far as try to specifically detect, if the message was issued by Delta chat, since it’s been getting some publicity on the web).
It’s probably worthwhile to screen the Spam folder (or just all folders, while pulling only the encrypted Delta messages) or add some other mechanism that would at least inform the chatters of what’s going on.