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If you try to send a blocked file type from Outlook, you will receive delivery failure message from the system. If someone tries to send you a blocked file type outside of Office Microsoft 365, their system may not block the message; however, Office Microsoft 365 will block the message and you won't receive their file.
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As a file is attached to a message, its size can inflate beyond the size of the attachment limit due to encoding. It is also possible that the receiving party, if outside of the Office Microsoft 365 system, may have a lower message size limit that will not allow larger attachments to make it through.
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