We recently became aware that some customers using Microsoft Outlook of various versions were experiencing missing email. Our technical team investigated and found the missing email's in the users Spam/Junk folders. The issue appeared to be localised to the last two months so we looked deeper and discovered that Microsoft had released an update to the Spam filter in Outlook in June. There are two issues that impact this, firstly some users due to their configuration cannot see the 'spam' and 'Junk email' folders without going into folder view, and secondly, even if you set the spam filter to 'None' it still in some circumstances takes action when it shouldn't.
One of the most significant issues that our customers have experienced with this 'change' is that email's between users on the same domain are being flagged as spam, when of course they are not. Surely, if fred@ sends to tom@ then the spam filter should leave well alone?
Further testing revealed that the updated spam filter was even more sensitive to spammy signatures (HTML Signatures that use external images etc) and that by removing the signature the email was passed.
The Spam filter can be disabled within the outlook settings but we have found, and users have reported that Outlook continues to filter regardless. Therefore we have found registry settings that can be applied to all three versions of outlook to permanently disable the spam filter, and this is our recommended option if you don't have the time or enthusiasm to educate the user base on how to manage Outlook's crazy spam filter. A link to these registry files can be found in our FAQ here. If you have office 365 then you can also find an article on how to disable its spam filter at the same link.