While currently porting VPE to 64-bit Windows, we experienced that there are some problems when sending mail via MAPI:
On a 64-bit Windows, a 64-bit application requires a 64-bit MAPI mail program and a 32-bit application requires a 32-bit MAPI mail program. This means you can not install Outlook for Win32 and call it via MAPI from your 64-bit app. But the greatest is: when you install Windows Live Mail (the successor of Outlook Express) on a 64-bit Windows 7, it is a 32-bit application. In other words, there is no 64-bit default mail client for Windows 64-bit. If you provide an e-mail button in your 64-bit application, make sure your customers have installed Outlook 64-bit. Otherwise they will receive an error message, even if they have Windows Live Mail installed. Congrats Microsoft... job well done!