I got this error ("550 Relaying denied. Please use SMTP Authentication") during e-mail sending. Other error that is visible: "553 sorry, that domain isn't in my list of allowed rcpthosts".
My configuration had the following entries in DNS records:
- SOA (Start of Authority)
- A (server's address)
- NS (nameservers that handle this domain)
- Additionaly reverse DNS was set properly
- Hostname used by server was the same as DNS name
Above records (in theory) should be sufficient to safely send an email. But it not worked for some recipients.
I have to add the following records to make sending e-mail pass:
- TXT "v=spf1 a -all" (Sender Policy Framework implementation)
Above TXT record defines IP from A record as the only allowed IP that can send emails from your DNS domain ("-all" flag is important when many conditions are defined, not the case here).