The above error means that the mail from domain.com was rejected due to spf setting. The sender domain: domain.comhave SPF setting that not allow to send the email from the IP x.x.x.x.
The spf is to prevent any mail forgery, spam issue, email spoofing and etc.
The basic SPF setting is v=spf1 a mx -all ,
[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.