Issue Problem to send email with Qmail 4.1.1

[DarkKikies]

The problem is isolated only to outgoing mail.
It is also not present on all outgoing emails but only in some with a personalized domain (like @domainxys.com) and @ libero.it or @ live.it email addresses. But not with @gmail.com and others.

Jul 1 06:40:35 cloud qmail: 1593578435.618013 starting delivery 10724: msg 910967 to remote xxxx@libero.it
Jul 1 06:40:35 cloud qmail: 1593578435.618088 status: local 0/10 remote 1/20
Jul 1 06:40:35 cloud qmail-remote-handlers[28887]: Handlers Filter before-remote for qmail started ...
Jul 1 06:40:35 cloud qmail-remote-handlers[28887]: from=xxxx@mydomain.com
Jul 1 06:40:35 cloud qmail-remote-handlers[28887]: to=xxxx@libero.it
Jul 1 06:40:35 cloud dk_sign[28888]: Starting the dk_sign filter...
Jul 1 06:40:35 cloud qmail-remote-handlers[28887]: PASS during call 'dd51-domainkeys' handler
Jul 1 06:40:35 cloud qmail: 1593578435.766643 delivery 10724: deferral: Sorry,_I_wasn't_able_to_establish_an_SMTP_connection._(#4.4.1)/

I tried all the possible guides, but with none I solved the problem.
As I said many outgoing emails arrive, and arrive without problems.

All the banchmarks of mxtoolbox are positive.


This is the source of an email on queue:

Received: (qmail 11210 invoked from network); 1 Jul 2020 21:35:49 +0200
Authentication-Results: cloud.mydomain.com;
spf=pass (sender IP is 127.0.0.1) smtp.mailfrom=name.surname@mydomain.com smtp.helo=webmail.mydomain.com
Received-SPF: pass (cloud.mydomain.com: connection is authenticated)
Received: from localhost (HELO webmail.mydomain.com) (127.0.0.1)
by localhost with ESMTPSA (DHE-RSA-AES128-GCM-SHA256 encrypted, authenticated); 1 Jul 2020 21:35:49 +0200
MIME-Version: 1.0
Date: Wed, 01 Jul 2020 21:35:49 +0200
From: name.surname@mydomain.com
To: tester.gino@libero.it
Subject: test invio email
User-Agent: Roundcube Webmail/1.4.6
Message-ID: <728d22084d89c2712d3ef8c8193ce0ee@mydomain.com>
X-Sender: name.surname@mydomain.com
Content-Type: text/plain; charset=US-ASCII;
format=flowed
Content-Transfer-Encoding: 7bit

 

[DarkKikies]

If I try to use telnet with smtp.libero.it on port 25 I can connect and communicate.

 

[Hex]

Maybe you can form an idea around the problem somehow.
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