E-mail sending "does not meet IPv6 sending guidelines regarding PTR records"

Hello, I'm using a Linode Server to send e-mails to my customers.
Almost everytime I send an e-mail to a Gmail address, I get rejected and receive this:

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gmail-smtp-in.l.google.com[2a00:1450:400c:c09::1b] said: 550-5.7.1
[2a01:7e01::f03c:92ff:fe65:ff33] Our system has detected that this
550-5.7.1 message does not meet IPv6 sending guidelines regarding PTR
records 550-5.7.1 and authentication. Please review 550-5.7.1
https://support.google.com/mail/?p=IPv6AuthError for more information 550
5.7.1 . f17si13788770wmb.19 - gsmtp (in reply to end of DATA command)
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I contacted Linode support and they sent me this:

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In this case, I did notice that it appears that there isn't a PTR record configured for 2a01:7e01::f03c:92ff:fe65:ff33:

2a01:7e01::f03c:92ff:fe65:ff33 dig -x 2a01:7e01::f03c:92ff:fe65:ff33 | grep 'NX'
;; ->>HEADER<<- opcode: QUERY, status: NXDOMAIN, id: 21103

Our rDNS guide also covers how to configure records for IPv6 addresses:

https://www.linode.com/docs/guides/configure-your-linode-for-reverse-dns/#ipv6-pools

I've also ahead and assigned the following /64 range to this Linode, to expedite any further troubleshooting you might need to do with IPv6 deliverability:

2a01:7e01:e001:284::/64 ? 2a01:7e01::f03c:92ff:fe65:ff33

You can now follow the instructions in our [Static IP configuration guide to set this up if needed:

https://www.linode.com/docs/guides/linux-static-ip-configuration/#configure-static-addressing

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I need someone to do this for me, please.