Hello,
I am using mail for Mac OS in Sonoma. I am not sure how or when but both my Macs (Studio and Air) suddenly have an strange email address after the remitent on every mail, this happens in 100% of my emails old and new as well as in the two Macs. Take a look:
This definitely looks suspicious so I would not email any files in response to this message.
FYI, it is easy to change the return address on an email. I could send you an email with a return address of President@whitehouse.gov but an examination of the message headers would reveal the real source of the message.
This is the message header ID in < >. You can turn it off by Settings > Viewing > Show Message Headers and change to Default. For some reason, mine had been changed to ‘Custom’. Hope that helps.