I had to disable warning warning C4389: '==': signed/unsigned mismatch
around the macro FD_SET.

This is the new code to disable this warning.

#ifdef SMTP_IS_WINDOWS
#pragma warning( push )
#pragma warning( disable : 4389 ) //warning C4389: '==': signed/unsigned mismatch
FD_SET(smtp->sock, &readfds);
#pragma warning( pop )
#else
FD_SET(smtp->sock, &readfds);
#endif