email - SPF and IPv6 fails even if the reverse DNS is created

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When sending mails to Gmail
I get this error

Received-SPF: fail (google.com: domain of test@domain.com does not

designate 2a01:4f8:xxx::2 as permitted sender)

client-ip=2a01:4f8:xxx::2;

When
I test the IPv6 2a01:4f8:xxx::2 reverse DNS ( href="https://network-tools.webwiz.net/reverse-dns.htm" rel="nofollow
noreferrer">https://network-tools.webwiz.net/reverse-dns.htm) it maps to
the correct host
smtp.domain.com

v=spf1
            a mx
            -all

• A
  record smtp targeting the IPv4
  88.133.xxx.xxx


• MX record
  targeting smtp
  

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class="normal">Answer

Looks
like you lack an AAAA record for smtp with content
2a01:4f8:xxx::2.