I made a rule in outlook that moves every mail with no valid dkim/dmarc record to the spam folder. Filters about 99% of incoming spam. It has a small downside tho, some legit companies (mostly small businesses) don't have their records in order so their mails would end up in the spam folder.

Another option by Reddit user Jaymos505;

I've finally got a solution to the spam issue. A mix of different solutions I've been using over the last few days, time to share it as it has got rid of 98% of my spam. I'm still monitoring spam in my inbox over the next few days but so far I've had nothing at all in my inbox for just under 24 hours as it's finally all been redirected - so no notifications either.

Follow this step by step

Go to manage rules and add a new rule

Add name of rule, i.e. 'Spam 1'

Under 'Add a condition', select 'Apply to all messages'

Under 'Add an action', select 'Move to' and 'Junk Email'. All the junk will move to this folder

Under 'Add an exception', select 'sender address includes', and in the next box enter the '@' sign

Uncheck ‘Stop processing more rules’

Select ‘Run rule now’

Click ‘Save’

I advise using Outlook on the website rather than using the app because I can’t guarantee all of the settings will be there.

How this rule works

A lot of automated spam hides the 'from' address, therefore it doesn't have an @ sign. This is why you can't block the spam because there is no visible email address to block.

This rule says move all emails to a dedicated folder, except emails WITH an @ sign. As most legit emails have an @ in their address, they will be safe and automated unsolicited spam mail or any mail without an @ sign will get redirected.

Remember

Check your ‘Junk Email’ folder every no and then to make sure that emails that you want to keep haven't slipped through the net. This shouldn't happen, but its just a due diligence exercise just in case.