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Back in 2019, Microsoft announced a big change to its list of blocked file types for attachments. At the time, Microsoft stated that it was doing so to improve security by adding "file extensions to the BlockedFileTypes property of existing OwaMailboxPolicy objects." The tech giant deems such file types as no longer secure for attachments and is a good way to protect customers from potentially malicious attacks from threat actors. Over time the company has expanded the block list for Outlook for the Web and also the New Outlook for Windows app. In a new message via the Microsoft 365 Admin Center portal, the company has announced that two more such file types will be added. However, the tech giant feels this should not affect most users, as these "newly blocked file types are rarely used." It writes: "As part of our ongoing efforts to enhance security in Outlook Web and the New Outlook for Windows, we’re updating the default list of blocked file types in OwaMailboxPolicy. Starting in early July 2025, the following file types will be added to the BlockedFileTypes list: .library-ms .search-ms" For admins, Microsoft says that no action is required if their respective organisations do not rely on the aforementioned file attachments. However, if they are still in use, then admins can "add them to the AllowedFileTypes property of your users’ OwaMailboxPolicy objects before the rollout." Speaking of which, this is set as a "major change" with the rollout expected to start in early July 2025. You can find information on how to do that in the official documentation on Microsoft's website here. For those wondering, the following file types are allowed by default: ".avi, .bmp, .doc, .docm, .docx, .gif, .jpg, .mp3, .one, .pdf, .png, .ppsm, .ppsx, .ppt, .pptm, .pptx, .pub, .rpmsg, .rtf, .tif, .tiff, .txt, .vsd, .wav, .wma, .wmv, .xls, .xlsb, .xlsm, .xlsx, .zip" You can find the list of blocked file types here on this Microsoft support article on the company's official website. If you are a system admin, you can view the message on the Microsoft 365 Admin Center under ID MC1090702. Source Hope you enjoyed this news post. Thank you for appreciating my time and effort posting news every day for many years. News posts... 2023: 5,800+ | 2024: 5,700+ | 2025 (till end of May): 2,377 RIP Matrix | Farewell my friend