Previously we were running Domino 8.5.1 FP 3 when suddenly our server system was down. It turned out that ID Vault was causing the issue, as documented here http://www-01.ibm.com/support/docview.wss?uid=swg21407899
We are required to upgrade the server to solve our issue. After we upgrade, the old problem solved and new problem came :(. All mail with attachment coming from internet will be automatically converted to .eml attachment. We submitted our issue to PMR and it turned out that the latest fixpack was causing the issue 852HF264_W32.exe.
So we need to uninstall this fixpack and still need to wait for IBM Support reply because our ID Vault problem is still not yet fix.
Lesson learned : don’t install fixpack 852HF264_32.exe after upgrading your Domino 852. But you MUST install 852HF16_W32.exe fixpack to avoid SMTP issue after upgrading your Domino to 852.