Undelivered Mail Returned to Sender gmail.com 550 5.7.1

Dear Sirs,

A few days ago, some messages sent to a mailbox with foward option return emails sended to domain gmail.com only.

The message invoke
"550-5.7.1 does not meet IPv6 sending guidelines regarding PTR records and
550-5.7.1 authentication. Please review 550-5.7.1"

The Sender domain only have IPV4 assigned
I try activate IPV6 on domain and create on DNS zone another PTR record with IPV6.

The problem occurs again.

Can you help me ?

Best Regards
Pedro Magalhães


Info:
OS...

Undelivered Mail Returned to Sender gmail.com 550 5.7.1