Details of a message

How to find more details (e.g. Subject, recipient, full From address, other headers) about a message such as you see indicated by a row in Detail Viewer, a report, or an alert

Overall the answer is: you cannot. DMARC XML reports do not contain any specific message information.  However here are some things you can try:
- Sometimes a report will show up in Forensic data https://dmarcian.com/forensic/ if your DMARC record requests reporting of RUF data. Note that few environments report this data; you may be able to improve this by asking your own inbox provider to send RUF data (and RUA if they do not yet! See our public reporters page to review who does currently participate in DMARC: https://us.dmarcian.com/dmarc-data-providers/ )
- ask folks in your Marketing and Sales groups if they are using a new vendor on this domain, and see if they can help track down messages based on what you see in the Detail Viewer
- Working in reverse from the last, it may be possible to ask the indicated vendor what the managing contact is at your company who is sending email using your domain