Problem sending email to Gmail recipients

Question: -------- Messaggio originale --------
Oggetto:
Undelivered Mail Returned to Sender
Data:
26/02/2024 09:26
Mittente:
[email protected] (Mail Delivery System)
Destinatario:
[email protected]

This is the mail system at host gierreweb-smtp.serverlet.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.

I get that error returned by Gmail:

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

: host gmail-smtp-in.l.google.com[66.102.1.27]
    said: 550-5.7.26 This mail has been blocked because the sender is
    unauthenticated. 550-5.7.26 Gmail requires all senders to authenticate with
    either SPF or DKIM. 550-5.7.26  550-5.7.26  Authentication results:
    550-5.7.26  DKIM = did not pass 550-5.7.26  SPF [simpitalia.com] with ip:
    [195.138.243.110] = did not pass 550-5.7.26  550-5.7.26  For instructions
    on setting up authentication, go to 550 5.7.26
    https://support.google.com/mail/answer/81126#authentication
    f16-20020adfdb50000000b0033cf35ca38fsi2143202wrj.1037 - gsmtp (in reply to
    end of DATA command)

Return-Path:
Received: from [192.168.1.55] (93-37-144-144.ip65.fastwebnet.it [93.37.144.144])
    by gierreweb-smtp.serverlet.com (Postfix) with ESMTPA id 338D7403FC
    for ; Mon, 26 Feb 2024 09:26:29 +0100 (CET)
Message-Id:
Mime-Version: 1.0
From: Segreteria SIMP
To: Sonia Di Carlo
Subject: The 47th Annual Foulkes Lecture and Study Day
Date: Mon, 26 Feb 2024 09:26:29 +0100
Content-Type: multipart/mixed; Boundary="--=BOUNDARY_226926_WNFW_CFSY_JMMQ_TTNW"

Answer: Hello. For a few weeks, Google Gmail no longer accepts unauthenticated emails anymore. You need to have a SPF, DKIM and DMARC records on your server DNS server.

Check your message for deliverability by clicking on the 'SpamCheck' button. The report will describe missing DKIM, SPF, and DMARC records.

You will find more information here: How to prevent mail to Gmail from being blocked.