It sounds like roundcube is working properly and that log entry says that sendmail queued the message for delivery. I think you are going to have to keep tracing the message through the delivery process to find out where its gone. Perhaps the sendmail community can help you.
SMTP debug:
[21-Feb-2019 09:48:09 +0300]: <8n0nfa0s> Recv: 220 *DOMAIN* ESMTP Postfix
[21-Feb-2019 09:48:09 +0300]: <8n0nfa0s> Send: EHLO localhost
[21-Feb-2019 09:48:09 +0300]: <8n0nfa0s> Recv: 250-*DOMAIN*
[21-Feb-2019 09:48:09 +0300]: <8n0nfa0s> Recv: 250-PIPELINING
[21-Feb-2019 09:48:09 +0300]: <8n0nfa0s> Recv: 250-SIZE 10240000
[21-Feb-2019 09:48:09 +0300]: <8n0nfa0s> Recv: 250-VRFY
[21-Feb-2019 09:48:09 +0300]: <8n0nfa0s> Recv: 250-ETRN
[21-Feb-2019 09:48:09 +0300]: <8n0nfa0s> Recv: 250-STARTTLS
[21-Feb-2019 09:48:09 +0300]: <8n0nfa0s> Recv: 250-ENHANCEDSTATUSCODES
[21-Feb-2019 09:48:09 +0300]: <8n0nfa0s> Recv: 250-8BITMIME
[21-Feb-2019 09:48:09 +0300]: <8n0nfa0s> Recv: 250 DSN
[21-Feb-2019 09:48:09 +0300]: <8n0nfa0s> Send: MAIL FROM:<admin@*DOMAIN*>
[21-Feb-2019 09:48:09 +0300]: <8n0nfa0s> Recv: 250 2.1.0 Ok
[21-Feb-2019 09:48:09 +0300]: <8n0nfa0s> Send: RCPT TO:<admin@*DOMAIN*>
[21-Feb-2019 09:48:09 +0300]: <8n0nfa0s> Recv: 250 2.1.5 Ok
[21-Feb-2019 09:48:09 +0300]: <8n0nfa0s> Send: DATA
[21-Feb-2019 09:48:09 +0300]: <8n0nfa0s> Recv: 354 End data with <CR><LF>.<CR><LF>
[21-Feb-2019 09:48:09 +0300]: <8n0nfa0s> Send: MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8;
format=flowed
Content-Transfer-Encoding: 8bit
Date: Thu, 21 Feb 2019 09:48:09 +0300
From: admin@*DOMAIN*
To: admin@*DOMAIN*
Subject: =?UTF-8?Q?=D1=84=D0=B0=D1=84?=
Message-ID: <59fe8aca337dfe66045682d73eda1cf3@*DOMAIN*>
X-Sender: admin@*DOMAIN*
User-Agent: Roundcube Webmail/1.3.8
test
[21-Feb-2019 09:48:09 +0300]: <8n0nfa0s> Send:
.
[21-Feb-2019 09:48:09 +0300]: <8n0nfa0s> Recv: 250 2.0.0 Ok: queued as D459022B95E1
[21-Feb-2019 09:48:09 +0300]: <8n0nfa0s> Send: QUIT
[21-Feb-2019 09:48:09 +0300]: <8n0nfa0s> Recv: 221 2.0.0 Bye