<win32/sendmail.h>
#define W32_SM_SENDMAIL_FROM_NOT_SET19

<win32/sendmail.c>
static char *ErrorMessages[] =
　{"\"sendmail_from\" not set in php.ini or custom \"From:\" header missing"}, // 19

/* Fall back to sendmail_from php.ini setting */
if (mailRPath && *mailRPath) {
　RPath = estrdup(mailRPath);
}
else if (INI_STR("sendmail_from")) {
　RPath = estrdup(INI_STR("sendmail_from"));
} else {
　if (headers) {
　　efree(headers);
　　efree(headers_lc);
　}
　*error = W32_SM_SENDMAIL_FROM_NOT_SET;
　return FAILURE;
}
つーことは、sendmail_fromが設定されていないんでねぇの？