Hit ⌘- (or go to Format->Style->Smaller) and then try printing. If some of your email is still cut off, make it smaller and try again.
The problem is that the "print" command in Mail prints what's displayed. A fixed-width html email should normally fit within the margins when printed, but if it's been forwarded, Mail crops the right side.
There are posts on here dating back to at least 2017 with this same issue. It's disappointing that this has never been addressed. Changing the paper size may work for some cases, but it could result in odd-shaped PDF files, and it's not helpful for printing.
Three suggestions for Apple:
Adjusting "scale" in the print dialog simply shrinks or enlarges the cropped email. But it's still cropped. Scale changes should have the same results as ⌘+ and ⌘- currently provide in this situation. An added benefit would be real-time viewing of the results.
The "scale" in the print dialog should be a slider. No one likes experimenting with various numbers until they get the right print size.
95% of the time I hit ⌘P it's to export a PDF. This is because the "export PDF" dialog box has no options, so you never know what the file will look like. It's useless, and confusing to people who don't know about "Print… Save as PDF". An alternative would be for ⌘P to be a "Print/Export…" dialog. With a slider for the scale.