The above error means that the mail from domain.com was rejected due to spf setting. The sender domain: domain.com have SPF setting that not allow to send the email from the IP x.x.x.x.
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The spf is to prevent any mail forgery, spam issue, email spoofing and etc.
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The basic SPF setting is v=spf1 a mx -all , means that
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[v=spf1]: This identifies the TXT record as an SPF string.
[a]: refer to the domain IP . The domain is allowed tosend mail through this A record
[mx]: refer to the domain MX record. The domain is allowed to send mail through this MX record
[-all]: No other servers are allowed to send mail from this domain.