sendmailコマンドを実行すると、DNS(?)で宛先が解決できず送信が失敗する

CentOS 9でSendmailコマンドを試しているのですが、スクリプトはエラーなく
実行できるものの、メールが送信できないという状況が続いております。
メール送信元、送信先はgmailのアドレスを使用しております。
お分かりになられる方、お力を貸していただけないでしょうか？

ShellScript(test1.gmail.com)
1#! /bin/sh
2{
3        echo "From: test1@gmail.com"
4        echo "To: test2@gmail.com"
5        echo "Subject: this is test"
6        echo
7        echo this is test.
8} | sendmail -i -t

Resultlog1(/var/log/maillog)
1Jan 30 23:27:28 localhost sendmail[48356]: My unqualified host name (XXX-XX-XXX-XXX) unknown; sleeping for retry
2Jan 30 23:28:28 localhost sendmail[48356]: unable to qualify my own domain name (XXX-XX-XXX-XXX) -- using short name
3Jan 30 23:28:28 localhost sendmail[48356]: 20UESSYi048356: from=root, size=90, class=0, nrcpts=1, msgid=<202201301428.20UESSYi048356@XXX-XX-XXX-XXX>, relay=root@localhost
4Jan 30 23:28:28 localhost sendmail[48356]: STARTTLS=client, relay=[127.0.0.1], version=TLSv1.3, verify=FAIL, cipher=TLS_AES_256_GCM_SHA384, bits=256/256
5Jan 30 23:28:28 localhost sendmail[48358]: STARTTLS=server, relay=localhost [127.0.0.1], version=TLSv1.3, verify=NOT, cipher=TLS_AES_256_GCM_SHA384, bits=256/256
6Jan 30 23:28:28 localhost sendmail[48358]: 20UESSu0048358: from=<root@XXX-XX-XXX-XXX>, size=311, class=0, nrcpts=1, msgid=<202201301428.20UESSYi048356@XXX-XX-XXX-XXX>, proto=ESMTPS, daemon=MTA, relay=localhost [127.0.0.1]
7Jan 30 23:28:28 localhost sendmail[48356]: 20UESSYi048356: to=test2.gmail.com, ctladdr=root (0/0), delay=00:00:00, xdelay=00:00:00, mailer=relay, pri=30090, relay=[127.0.0.1] [127.0.0.1], dsn=2.0.0, stat=Sent (20UESSu0048358 Message accepted for delivery)
8Jan 30 23:28:29 localhost sendmail[48360]: STARTTLS=client, relay=gmail-smtp-in.l.google.com., version=TLSv1.3, verify=OK, cipher=TLS_AES_256_GCM_SHA384, bits=256/256
9Jan 30 23:28:31 localhost sendmail[48360]: 20UESSu0048358: to=<test2.gmail.com>, delay=00:00:03, xdelay=00:00:03, mailer=esmtp, pri=120311, relay=gmail-smtp-in.l.google.com. [IPv6:2404:6800:4008:c13:0:0:0:1b], dsn=5.0.0, reply=550 5.7.1 [2400:8500:1302:772:118:27:107:171      12] Our system has detected, stat=Service unavailable
10Jan 30 23:28:31 localhost sendmail[48360]: 20UESSu0048358: 20UESVtx048360: DSN: Service unavailable

resultlog2(/var/spool/mail/root)
1From MAILER-DAEMON@ドメイン名  Sun Jan 30 23:28:31 2022
2Return-Path: <MAILER-DAEMON@ドメイン名>
3Received: from localhost (localhost)
4        by ドメイン名 (8.16.1/8.16.1) id 20UESVu0048360;
5        Sun, 30 Jan 2022 23:28:31 +0900
6Date: Sun, 30 Jan 2022 23:28:31 +0900
7From: Mail Delivery Subsystem <MAILER-DAEMON@ドメイン名>
8Message-Id: <202201301428.20UESVu0048360@ドメイン名>
9To: postmaster@ドメイン名
10MIME-Version: 1.0
11Content-Type: multipart/report; report-type=delivery-status;
12        boundary="20UESVu0048360.1643552911/ドメイン名"
13Subject: Postmaster notify: see transcript for details
14Auto-Submitted: auto-generated (postmaster-notification)
15
16This is a MIME-encapsulated message
17
18--20UESVu0048360.1643552911/ドメイン名
19
20The original message was received at Sun, 30 Jan 2022 23:28:31 +0900
21from localhost
22with id 20UESVtx048360
23
24   ----- The following addresses had permanent fatal errors -----
25<root@118-27-107-171>
26    (reason: 550 Host unknown)
27
28   ----- Transcript of session follows -----
29