How to setup local email service inside a virtual server with roundcube software

it’s not working
i tired to send out many emails to test n non of them was received by the other end.

and after 5 days of sending these emails out, i got a reply email for each of them looks like this

This is the mail system at host server. domain. net.

I’m sorry to have to inform you that your message could not
be delivered to one or more recipients. It’s attached below.

For further assistance, please send mail to postmaster.

If you do so, please include this problem report. You can
delete your own text from the attached returned message.

               The mail system
: delivery temporarily suspended: connect to alt2 .gmail-smtp-in.l .google.com[2607:f8b0:4023:c0b::1a]:25: Network is unreachable Reporting-MTA: dns; server. domain. net X-Postfix-Queue-ID: DBDE6BF909 X-Postfix-Sender: rfc822; info @domain.com Arrival-Date: Fri, 17 Sep 2021 23:59:56 +0000 (UTC)

Final-Recipient: rfc822; title @gmail.com
Original-Recipient: rfc822;title @gmail.com
Action: failed
Status: 4.4.1
Diagnostic-Code: X-Postfix; delivery temporarily suspended: connect to
alt2 .gmail-smtp-in.l .google.com[2607:f8b0:4023:c0b::1a]:25: Network is
unreachable
Return-Path: <info @domain.com>
Received: from domain.com (localhost [127.0.0.1])
by server. domain. net (Postfix) with ESMTPA id DBDE6BF909
for <title @gmail.com>; Fri, 17 Sep 2021 23:59:56 +0000 (UTC)
MIME-Version: 1.0
Date: Sat, 18 Sep 2021 02:59:56 +0300
From: “info @domain.com” <info @domain.com>
To: title @gmail.com
Subject: Testing
User-Agent: Roundcube Webmail/1.4.11
Message-ID: 387bac7e5f5a8660099be0f5eaab177d@domain.com
X-Sender: info @domain.com
Content-Type: text/plain; charset=US-ASCII;
format=flowed
Content-Transfer-Encoding: 7bit

Reporting-MTA: dns; server. domain. net
X-Postfix-Queue-ID: DBDE6BF909
X-Postfix-Sender: rfc822; info @domain.com
Arrival-Date: Fri, 17 Sep 2021 23:59:56 +0000 (UTC)

Final-Recipient: rfc822; title @gmail.com
Original-Recipient: rfc822;title @gmail.com
Action: failed
Status: 4.4.1
Diagnostic-Code: X-Postfix; delivery temporarily suspended: connect to
alt2 .gmail-smtp-in.l .google.com[2607:f8b0:4023:c0b::1a]:25: Network is
unreachable