Boss's E-mails are coming in garbled and marked "[Unknown Sender]"

As some background, my boss uses the Old Outlook on his PC and is unlikely to change that. BUT we've had enough problems over the years that we have him keep a Gmail tab open.

Now, I am the most tech literate person in the office, but I'm by no means a computer scientist. So, I'll try to describe it the best I can.

He had me look at his emails because it was getting these strange garbled e-mails with unknown senders. I saw many lines of what looks like raw HTML and Hexadecimals lines of code. He deleted the first two, assuming they were spam. The third one, garbled in Outlook but when viewing it on g-mail, was a photo from his assistant. Perfectly normal. The 2 the deleted, when looking in the trash via g-mail, were still garbled though. Something about deleting them in Outlook is making them permanently unreadable.

We have restarted the computer and it seems to be persisting. Any clue what this is?