I would be surprised if there are more working email clients out there than working 3D engines. The gaming market is huge, most people do not pay to use email, hobbyists love creating game engines.
Idk, a working basic email client is just not that hard to write though. SMTP and IMAP are simple protocols and the required graphical interface is a very straightforward combination of standard widgets.
I don't mean to be contrarian, but this is completely false.
IMAP _seems_ to be a straightforward (but nasty and stateful) protocol, until you find out that every major provider ignores RFCs and does things slightly differently.
