Fail to send Email to Gmail ....

I’ve actually configured postfix as on this tutorial: How to Install and Configure Postfix as a Send-Only SMTP Server on Ubuntu 16.04

Then, I tried Step 3 to send an Email to my Gmail account, as follows:

 echo "This is the body of the email" | mail -s "Gmail Test" myAccount@gmail.com

But, I always obtained the following ERROR message:

▸N 43 Mail Delivery Syst 2019-02-17 12:43   80/3112  Undelivered Mail Returned to Sender                                                                                                                      
? 
[-- Message 43 -- 80 lines, 3112 bytes --]:
From MAILER-DAEMON@mail.longervision.com Sun Feb 17 12:43:27 2019
Date: Sun, 17 Feb 2019 12:43:27 +0000 (UTC)
From: MAILER-DAEMON@mail.longervision.com (Mail Delivery System)
Subject: Undelivered Mail Returned to Sender
To: doaccount@do-computer
Message-Id: <20190217124327.3665B17A418@mail.longervision.com>

[-- #1.1 23/933 text/plain, 7bit, us-ascii --]
[-- Notification --]

This is the mail system at host mail.longervision.com.

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

<myAccount@gmail.com>: host gmail-smtp-in.l.google.com[74.125.20.27] said:
    550-5.7.1 This message does not have authentication information or fails to
    pass 550-5.7.1 authentication checks. To best protect our users from spam,
    the 550-5.7.1 message has been blocked. Please visit 550-5.7.1
    https://support.google.com/mail/answer/81126#authentication for more 550
    5.7.1 information. d65si10644912pfc.201 - gsmtp (in reply to end of DATA
    command)

[-- #1.2 20/806 message/delivery-status, 7bit, us-ascii --]
[-- Delivery report --]

[-- No MIME handler installed, or not applicable --]

[-- #1.3 15/507 message/rfc822 --]
[-- Undelivered Message --]

Subject: Gmail Test
To: <myAccount@gmail.com>
Message-Id: <20190217124326.D601A17A3D4@mail.longervision.com>
Date: Sun, 17 Feb 2019 12:43:26 +0000 (UTC)
From: XXXXX <doaccount@do-computer>

This is the body of the email

Can anybody please give me a hand…

Cheers Pei