Email newsletter not readable

Hi: I got a strange question from one of my paid users for my newsletter who uses Apple Mail on iMac:

“There seems to be a change in your email formatting the last two weeks? I cannot scroll down the email if I open it. I can only read the whole thing either online or when I respond or forward the email like I am now.”

This only seems to be impacting him. It certainly doesn’t show up on any test emails I send to myself and I polled several other folks and they’re not having this issue.

Any theories on why this might be happening?