domain name system – Allowing Messages from IP Address DMARC

I’m still learning infrastructure stuff and need to ask a question which relates to DMARC. A user in the organisation has an app which is sending marketing messages to staff within the business. The messages leave the network and come back in, but are detected as SPAM.

I have had a look and it seems that we would need to get the IP address of the source to be whitelisted through DMARC.

We have a DMARC record on external DNS and we have a DKIM policy. I can verify this from a combination of using online checker tools, the message headers from an email and looking at the DNS records. Does this mean that in order to allow the messages from the user’s app, I will only need to add the IP address of the source to the SPF record?

Is there more steps involved? If so, would someone be king to point me in the right direction please?