Mail cut off after 5 years of life on the VPS. Exim. How?

Question

Exim

Hello.
I apologize if this is in the wrong section, but I couldn't find a better one.
There is a VPS and several sites on it. Not so long ago, I transferred one of the sites to another server and deleted it on the one that now has problems. Whether there is a relationship between the removal of one domain on the VPS and the suspension of mail - I do not know, but there were no other events.
On the VPS, the server name is one of my domains, let's call it mysite.ru. This is the name of the server, such a site lies on it and there is only one mailbox for this domain: [email protected] All!
There is another site on this server, I will call it osnovnoy.ru, since it is the most lively and I need it. So, I send a letter from the mailbox [email protected] to my third-party mailbox on mail.ru, the answer comes:
-------------
This message was created automatically by mail delivery software.
A message that you sent could not be delivered to one or more of its
recipients. This is a permanent error. The following address(es) failed: my other
[email protected]
retry timeout exceeded
------------
The letter, accordingly, does not reach anywhere.
I climb into the Exim logs, and specifically into the mainlog, and there ... miraculously wonderful and wonderfully wonderful:
Code
2019-09-03 23:12:01 Warning: purging the environment.
Suggested action: use keep_environment.
2019-09-03 23:12:01 Warning: purging the environment.
Suggested action: use keep_environment.
2019-09-03 23:12:01 1i5CLZ-0005S5-F5 <= [email protected] U=root P=local S=821 from for root
2019-09-03 23:12:01 1i5CLZ-0005S5-F5 ** [email protected] R=unknown_users: Unknown user
2019-09-03 23:12:01 1i5CLZ-0005SA-GH <= [email protected] U=root P=local S=738 from for root
2019-09-03 23:12:01 1i5CLZ-0005SA-
GH 1i5CLZ-0005SE-Nr <= <> R=1i5CLZ-0005S5-F5 U=Debian-exim P=local S=1591 from <> for [email protected]
2019-09-03 23:12:02 1i5CLZ-0005SE- Nr ** [email protected] R=unknown_users: Unknown user
2019-09-03 23:12:02 1i5CLZ-0005SE-Nr Frozen (delivery error message)
2019-09-03 23:12:02 1i5CLZ-0005S5-F5 Completed
2019-09-03 23:12:02 1i5CLZ-0005SI-Ur <= <> R=1i5CLZ-0005SA-GH U=Debian-exim P=local S=1508 from <> for [email protected]
2019-09-03 23:12:02 1i5CLZ-0005SI-Ur ** [email protected] R=unknown_users:
Unknown
user -09-03 23:12:02 1i5CLZ-0005SA-GH Completed
Those times!
Firstly, what is [email protected], above I wrote that physically on the server to this domain there is only [email protected]
Secondly, how did everything work for 5 years and what happened now?
Guys, where to dig?
PS Letters are not sent from any mailbox of domains on the server, incoming mail also does not arrive.

Answer 1

By the way, I checked now, the mail safely leaves and arrives between the boxes on my server. There is no access to the "open sea".