Using Classic Outlook on Windows for personal email, calendar, and contact management
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However, I still can't get it to work with this one message to which I'm trying to reply.
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Have no idea as to what "with this one message" refers to so can't provide any info beyond that already provided beyond side-stepping the reading pane as the starting point and opening the message and then clicking <reply> - yes this involves an extra step but as mentioned - no idea what's unique about the specific message in question. (is this a message rec'd in "text format" versus "HTML" by any chance?)
Please note that your original post did not refer to a 'specific message" to which you indicated that my response did not resolve your issue - there was no way for anyone to know that.