macOS Mail Rules – How to Apply Rules to Past Messages in Outlook

Many mail servers have their own spam filters and will add a special header to the message to indicate spam (some are yes/no, others will use a number system where higher numbers indicate the more likelihood the message is spam)

Select a message and Select View -> Message -> Raw Source from Mails menu to see all the headers and info. You may see something like this (highlighted):

Based on what you find, you may set up a rule based these headers. (in Mail -> Preferences -> Rules, you may need to add the header in mail (see img))

Repeat this for each Mail client.

As for Mail.app's custom heuristics... That's a bit trickier, but if you are consistent they should end up pretty close on each machine. (I tend to trust my server's filtering more anyway)

The action of marking an email as read forces the rule to be a "client-only rule", which means it will work only when MS Outlook is running (and will be done by MS Outlook and not by MS Exchange).

You can (re)confirm if this rule is client-only by deleting it and creating it again in MS Outlook. If you see a warning like the one shown here, then it will only work while MS Outlook is running.

When you look at the rules you have in MS Outlook (from the "Manage Rules & Alerts" menu option), client-only rules will show "(client-only)" against the rule name.

Also see Managing Inbox Rules in Exchange Server