They have to update their SPF records in their DNS Server, which is the solution for this problem.
Optionally, from our side, we can whitelist the sender in /etc/spamdyke/whitelist_senders From: ChandranManikandan [mailto:kand...@gmail.com] Sent: 27 June 2019 14:45 To: firstname.lastname@example.org Subject: [qmailtoaster] 451 SPF lookup failure (#4.3.0) Hi Friends, One of our sender try to send an email to our domain but they got bounced back with this message 451 SPF lookup failure (#4.3.0). I am running qmailtoaster in centos 6 32 bit. We are able to send email to this sender but unable to receive it and they got bounced back email. Is the issue is our server or their server. how to fix this Anyone can help me. -- Regards, Manikandan.C