I’m also looking for an answer. My WL Mail blocks images in some messages only, pretty much like how Hotmail/Outlook.com used to work, although Outlook.com does not have this problem anymore.
Anecdotally, it seems that the issue have something to do with the X-Message-Delivery header, which Hotmail/Outlook.com adds to inbound messages—it is my observation that the vast majority of messages have the value V=1.1;us=0;l=1;a=1;D=1;GD=1;SCL=0, while those few that do have images blocked have V=1.1;us=0;l=0;a=1;D=1;GD=1;SCL=0. (The actual values are base64-encoded.)
While it is not a valid proof, you may want to check if …
- … messages with which image blocking works have an X-Message-Delivery value that begins with Vj0xLjE7dXM9MDtsPTA7; and
- … messages with which image blocking does not work have an X-Message-Delivery value that begins with Vj0xLjE7dXM9MDtsPTE7.
If this is indeed how it works, I guess only Microsoft will be able to fix this problem. (I tried tweaking %LocalAppData%\Microsoft\Windows Live Mail\SAS.Rule but the rule set SafetyLevelDetermination does not seem to affect image downloading behavior.)
P.S. The Windows 8 Mail app (version 16.4.4406.1205) has a similar issue—images may appear blocked, but downloading still occurs (cf. http://answers.microsoft.com/en-us/windows/forum/windows_8-ecoms/windows-8-mail-loads-external-images-even-if/4d777c80-3922-41b9-af0f-7dba938eae98).