There are instances in which you may be searching for messages that are related to each other in Outlook, but cannot locate them. There is a feature available to help with this.
1. In a list of email messages, right click on the message in in which you are looking for related messages.
2. Choose "Find Related."
3. Choose either "Messages in the Conversation" (related to topic) or "Messages from Sender" (related to person).
This file, along with illustrations can also be found in SharePoint > Software Users Group > Shared Documents > Outlook-Office 365 > Finding Related Messages.