Why does mail keep coming to the old mailbox after changing the MX record?

Question

Domain Name System

The point is the following. Once upon a time I had a domain.
The domain had an mx record, in which the hosting was registered, where the site spins. But the mail server there was bad, so I thought, and transferred the mail to Yandex (to Yandex.Connect).
I followed exactly the instructions and created a 10 mx.yandex.net record for my MX domain.
I registered DMARC, DKIM, everything seems to be fine. Almost all letters now come to a new mailbox on Yandex.
The key word is "almost". For some reason (and I'm at a loss as to why), some letters continue to come to the old mailbox (which was on the hosting). MX record checked already 100500 times. I pierced the mailbox with external utilities, tested MX, everything points to mx.yandex.net, but for some reason all the same, some of the letters continue to stubbornly come to the old mailbox.
What can be wrong?

Answer 1

The RFC says the following. https://datatracker.ietf.org/doc/html/rfc7505
First, the mailer takes the MX record, if the mail does not go through, then it uses the A record. That is, for any errors, the remote mailer will use the A record.
I assume that Yandex SPAM does not let some letters through and they will try to send them to the A record.
The same RFC says to add a null MX record to prevent this from happening.
Something like
MX 0 "."

Answer 2

Cache dns-records from some providers. Check via https://dnschecker.org/

Answer 3

from only one sender

Look in his letters Received.