Since updating to OS X 10.9 I can't print my emails.

You "can't print some of your emails" ... which must mean you can print the rest.

So what defines the ones that don't print? What attribute do they have in common?

All mine print just fine.

From Mail's menu, check the line that says Edit > Attachments > Always Send Windows-Friendly Attachments

i have the same problem. i have 10.9 osx. use the mail app. some emails don't print in their entirety. for instance i have a receipt from apple that only prints the to: from: info and not the body of the email. under edit-attachments-always send windows friendly attachments....mine is already checked. i unchecked and rechecked. still wont print. help please.

I had the same problem. Most would print, but I tries printing a receipt from Paypal and it wouldn't. I simple changed from portrait to landscape orientation and it worked. WEIRD! This is definitely a bug.

"Resolved" it by defining a custom paper-size that is a bit shorter than A4 (210x293mm) -

1. Choose the mail-message and press [Command]+[p] to get the print-settings screen
2. Choose the Printer
3. Choose Paper Size -> "Manage Custom Sizes...", then, in the "Manage Custom Sizes" screen:
4. Press the [+] sign to add a new custom-page-size
5. Click the "Untitled" list-item to edit its name to something like "A4_iMac" or whatever
6. Non-Printable Area: Choose your printer (this also sets the Paper Size)
7. Fix the Paper Size to 210mm (width) by 293mm (height)
8. Press [OK] to save your changes
9. Back in the Print-Settings page, choose the newly-created Paper Size - your mail-message should be showing in the preview, now.

Next time, simply choose that custom-made paper-size before printing

(or even make it the default, until this is resolved).