One way is to export your mail folder contents to individual email messages in .eml format. Then download the email viewer at https://www.systoolsgroup.com/eml-viewer.html - free. The addon importexporttools can export messages to eml by highlighting the folder, rightclicking and selecting the importexport option to export as eml files . The product I show is an example; I'm sure there are other eml viewers out there. Another option is to read the mail folder directly with an mbox viewer, one of which is https://www.nucleustechnologies.com/mbox-viewer-free.html