Can't find specific messages from a particular email thread? Conversation View might be the issue!
In Outlook Web App (OWA), conversation view groups email messages into conversations based on the subject of the message. Conversation view is enabled by default for every mail folder in your mailbox. You can disable it, but you will need to disable it on each folder separately; there is no option to disable the setting on multiple folders.
To disable or enable conversation view:
Please note: This issue does not occur in Outlook 2010 or 2013.
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