Why does Yandex.Mail give errors when fetching a message from a subdomain?

Question

MySQL

Good day
Added a postman type subdomain to mailgun. domain-name.com . The domain is purely for work with mail. Created 5 DNS records that they asked for verification. The domain was confirmed and connected to the service.
Now, I send letters from it to my mailbox created in Yandex.Mail.
If the "From" field contains the address [email protected] , the letter appears in the Yandex mailbox.
If the 'From' field is filled in as [email protected] , then the message is not delivered, and in the mailgun interface it is marked with an error:

5.2.2 Mailbox size limit exceeded 1578276298-gyMZMkM6XP-4vS4AhHG

A search on this message brings up a Yandex page, where they write that you need to enter the mail web interface in order to fix this error. Okay, I open the mail web interface, send an email from [email protected] and see it in my inbox. Immediately, without closing the browser tab with the mail interface, I change the address in my application to [email protected] - the letter does not arrive, the error cited above is in the mailgun log.
Why is this happening? I don't have any ideas at all...
If you ignore and leave [email protected] as the sender, then MS Outlook throws emails in SPAM, because mail from Google via G Suite is connected on the main domain
Update 1 : Tried sending via both HTTP API and SMTP. The result is the same in both cases.
Update 2 : The error only pops up if the domain was created in the US/North America region. For domains from the Europe region, this problem does not appear. What is it, if not sanctions ... ha ha ((

Answer 1

The error message says that the mailbox limit has been reached. Or the place is over or the size of the mailing is exceeded.