|I'm not really familiar with the stuff you're using, and I haven't used C++ for a few years, but from what I remember any functions in the Windows API that changed between character types depending on precomiler definitions had other similarly named functions.
I remember the function to show a message box was called MessageBox(), and internally it mapped to either MessageBoxA() or MessageBoxW() depending on precompiler definitions.
Maybe you'll find something similar with your function for sending the mail?
Then depending on how speedy you need it to run you could write some code to attempt the other version of the function if the first try failed, or if you need it faster try to detect what the MAPI client needed you to use and have some kinda fancy interface, function pointer, etc to convert your text and send it using the right format.
Alternatively, you could try using .Net from C++ to use all of the email junk built into that - though I'm kinda assuming you're avoiding needing to install the entire .Net framework alongside your application.