Hi.
One of our clients was expecting an email, but the server rejected it for the following reason:
2019-12-30 10:16:02 SPFCheck: 1.2.3.4 is not allowed to send mail from mailserver.tld: Please see http://www.open-spf.org/Why?id=recepient%40mailserver.tld&ip=1.2.3.4&receiver=our.server.tld ...