What is the most used and flexible SMTP Client library in C/C++? [closed]

Answer 1

libquickmail has all the features you need. It supports things like multiple alternative bodies and attachments.

Answer 2

My personal favourite is VMime, for C++ only, but the highly reputed libcurl also has SMTP support (as well as many other features).

VMime has a dual license; I think curl has a sort of MIT-style license.