I really am not sure how this information might relate to e-mail at all.
after a quick bit of googling it seems to me that the information provided by Microsoft is related to software you/someone in your organisation has installed on a windows machine, which was originally signed by Bespoke Software Solutions LImited, and that the certificate that was used for signing (belonging to Bespoke Software Solutions) was compromised at some point:
* you need to find the software signed by Bespoke Software Solutions on your Windows Endpoint (PC) and remove it since there might have been a compromise
AFAICT this has nothing to do with emails going through PMG?