I activated the SPF on my mail server. To ensure that it is working, applying the test to send a standard email to the <code> check-auth@verifier.port25.com </ code>, it returns with the status "SoftFail".
SoftFail: "The sender's ADMD believes the client was not authorized to inject or relay mail using the sender's DNS domain, but is unwilling to make a strong assertion to that effect."
With each new test e-mail, it sends with a different IP address.
How to solve?