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We have a domain on our SmarterMail and it's working fine.
Now we have a 3 party service that are sending mails via SendGrid on behalf of that domain and that part are also working. All SPF settings on the domain are in place.
But when we have an incoming mail to our SmarterMail from SendGrid the mail are rejected by SmarterMail with
"rsp: 550 Authentication is required for relay"
We have asked the 3 party service if they are sending mail from a static IP, so that we could put that into the SMTP relay. But they do not have that.
How do we solved this issue so we can around the error?
And should SmarterMail not look at the SPF record and know that the sender (SendGrid) is a trusted sender?
Hope someone can help on this issue