You don't write how large volumes you send, and how your sending pattern is (e.g. equal volume per day, or biweekly spike) - also, how many percent of your email volume go to Microsoft inboxes (i.e. Live.com, Hotmail.com, Outlook.com, Msn.com)? So it is not easy being specific. I obviously do assume you follow best practice for managing opt-outs, not buying your lists, etc.
This sight is not uncommon, when it comes to Microsoft inboxes, as their spam filter is basically rubbish, and an eternal source to many of the grey hair you find on my head. There are a two ways in which you can try and resolve this issue. One is going directly to Microsoft, the other one is logging a support case with Salesforce.
I am a huge fan of using SNDS, which gives me an insight into how Microsoft is treating emails from my IP address. You can read more about it here. You will need to involve Salesforce in gaining access. Here is how.
Once you have access, you will be able to see the status of your IP, and the volumes received by Microsoft across their domains. This is how a healthy IP with low volume should look like:
In this example you see low daily volumes (probably transactional emails) and few occasional campaigns.
You don't want you SNDS to look like this:
Any red or yellow "days" are bad for you, and once you have more than a week of consecutive reds, regaining green status is really hard without actively involving Microsoft. This can be done by filling out this form with information about your sender.
Be prepared, that the initial response from Microsoft will be a standard text:
We have completed reviewing the IP(s) you submitted. The following
table contains the results of our investigation.
Not qualified for mitigation
Our investigation has determined that the above IP(s) do not qualify
Please ensure your emails comply with the Outlook.com policies,
practices and guidelines found here:
To have Deliverability Support investigate further, please reply to
this email with a detailed description of the problem you are having,
including specific error messages, and an agent will contact you.
You need to be persistent, and keep pushing on, asking for the case to be escalated. That is of course, given you do follow best practice for sending marketing email.
One good thing to remember, before starting sending emails from a new IP/SAP is the option of pre-emptive accommodation process. This basically prepares Microsoft for knowing, that you are about to start sending legitimate emails from a new IP address. This might be too late for you, but could be worth trying. Validity (Return Path) describes this process quite well in this article.
If you prefer going through Salesforce, you can ask for their help in reestablishing good reputation with Microsoft by logging a case.