Recipients get "unverified" emails with "CAUTION: SUSPECT SENDER" warnings

Hi,

Does anyone have a solution to this issue - see subject?

All our self-authentication is in place and green results. DNS records are all correct. I have been onto Microsoft and they are saying SPF softfail the likely cause. But, CC KB says it is not required which the check will not validate due to sender (ours) and sending system (CCs) both being different domains. I have requested our domain management to add it nonetheless.

MS Support also said to submit an example email into their analysis system in MS Security portal.

Interestingly, it has come back with:

Also, the Sender IP is showing as a Microsoft IP address. Also, the Message Header Analyzer (https://mha.azurewebsites.net) shows this for the Received-SPF:

SoftFail (protection.outlook.com: domain of transitioning in.constantcontact.com discourages use of 104.47.17.108 as permitted sender)

We would just like to get clean messages out to recipients for our events rather than getting blocked inadvertently.

Thanks.

Jason