The message trace feature within Exchange Online works pretty well but can be a challenge if you want to search based on a particular email subject. In a scenario where you want to know who received an email or a set of emails, you have to employ some tricks to be able to query large amounts of logs.
The script allows you to search on a subject or variants of a subject going back X number of days. The output is logged to a CSV file showing the details of the trace log entry.
Additional information on the usage of this script can found at the following blog post: Office 365 - Script to Perform Message Trace By Subject