What the message looks like
550-5.7.25 The IP address sending this message does not have a PTR record setup, or the corresponding forward DNS entry does not match the sending IP550 5.7.25 Reverse DNS validation failed
What it means
The IP address of the sending server has no reverse DNS (PTR) record, or its PTR record points to a hostname that doesn't resolve back to the same IP address. The standard name for this status is "reverse DNS validation failed".
Gmail and many other providers require valid reverse DNS before they accept email, because servers without it are very often compromised machines sending spam.
If you're the recipient
There's nothing to change on your side. Let the sender know their emails are being rejected.
If you're the sender
Reverse DNS is set by whoever controls the IP address, which is usually your hosting provider or internet provider, not your domain's DNS. Ask them to set a PTR record for your server's IP address, such as mail.example.com, then make sure that hostname has an A or AAAA record pointing back to the same IP. Use the same hostname in your server's HELO.
Home internet connections usually can't send email directly for this reason. Send through your email provider's SMTP server instead.
If you use addy.io
addy.io rejects email from servers without valid reverse DNS with 554 5.7.1 Client host rejected: cannot find your reverse hostname. See 554 5.7.1. The sender has to fix this, not you.