As different platforms (operating systems) use different format of text files, many transfer protocols support special mode for transferring text files (called text or ASCII mode). In the mode the file is not only transferred, but also converted to format used by the target platform. The mode must not be used for binary files (including document files from modern text processors, like Microsoft Word), as they must be transferred without modification.
So, in order to make a proper backup, configure your account in a mailclient that supports making a backup. Just because OWA does not support it, doesn't mean its not possible. In modern mail clients such as MS Outlook and Thunderbird, just entering your email address and password automatically configures the client using autodiscovery. Not going for a mail client to make a backup and strictly limiting ones self to OWA is not necessary.
For those with a modern (GNU) version of chmod you may be able to do this all in one step. Symbolically this equates to "everyone (group/other) has the same as the owner, but remove write permissions from group/other"
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