C :: Convert List Of Names From One Text File To Email Format In New File

I have almost a hundred names in a text file that I want to convert to email addresses and save to another file. I seem to have it working, but it doesn't print the full names in the email prefix. The output I'm looking for is Doe_John@livebrandm, but I'm only getting D_J@livebrandm. I'm not sure what I should specifically be reading up on that applies to this directly.

Code:

#include <stdio.h>
#include <stdlib.h>
int main(void)
{
FILE *fpin=fopen("namesIN.txt", "r");
FILE *fpout=fopen("emailOUT.txt", "a");
char first[20],last[20],inbuff[1500];

[Code]...