When having an email account that's on another server that forwards to the MS-protected server,
MS will assumedly pass through all spam because the forwarding server IP is not abusive.
This suggestion is to be able to "register" (special-whitelist) an IP as "transparent" such that MS, peels off the forwarding IP address and then considers the next IP in the header as the sender for the purposes of IP evaluation.
Otherwise, forwarded spam from another server do not get considered for filtering.
In my own case, i have forwarders on about 10 other servers that are not mine to control or install MS, and thus i'm wide open for spam forwarded thru those IP addresses.
Note that AOL and Comcast, for instance, when considering their FBL "report spam" do NOT consider domain-account-forwarded emails and thus the hosting account's server gets reported as the spammer, when it's just the conduit because of the forwarder.