Validando o formato do email informado

Nesse exemplo é passado como argumento um email e validado se não possui nenhuma restrição imposta para caracter inválido e formato.

#include < stdio.h >
#include < stdlib.h >
#include < unistd.h >
#include < string.h >
#include < ctype.h >

int caracter_invalido_email(char * email)
{
   char * walker = NULL;
   char *caracter = "#$%¨*()+=-/´`^~[]{}<>;;:?";
   int i;
   walker = email;
   while ((walker != NULL) && (*walker != '\0'))
   {
       for(i = 0; *(caracter+i) != '\0'; i++){
          if(*walker == *(caracter+i)){
              printf("Caracter invalido encontrado %c\n", *(caracter+i));
   
              return -1;
          }
       }

       walker++;  
   }

 return 0;
}

void consist_email(char * email)
{
 char *walker = NULL;
 int size;
 int found_at;
 int found_dot;
 int found_last_dot=0;

 found_at = found_dot = -1;
 size     = 0;

 printf("Iniciando a função \n\n");
 walker = email;
 printf("Posicao do Ptr %c \n\n",*walker);

 printf("Walker %s \n\n", walker);
 while ((walker != NULL) && (*walker != '\0')) 
 {
     printf("Entrando no while %c \n\n",*walker);
     if (*walker == '@') 
     { 
         /* Search for @ */
         printf("Iniciando walker==@ \n");
         if (found_at == -1)
     {
     printf("Iniciando walker==@ found at==-1 \n");
     found_at = 0;
     }else{
          /* only one @ */
          printf(" ERRO MAIS DE UM @ \n");
          return;
     }
     if(*(walker+1)== '.'){
         printf("ERRO . after @ \n");
         return;
     }
     if(*(walker-1)== '.'){
         printf("ERRO . before @ \n");
         return;
     }

  }
     if (*walker == '.') 
     { 
       if(*(walker+1)== '.'){
           printf("ERRO . after . \n");
           return;
       }
       if(*(walker-1)== '.'){
           printf("ERRO . before . \n");
           return;
       }

     }
     if (*walker == ' ') {
         /* no space allowed */
         printf("ERRO NENHUM ESPAÇO É PERMITIDO\n");
         return;
     }
  

     /*Não pode letra maiúscula*/
     if (isupper(*walker))
     {
         printf("upper case letter not allowed\n\n");
         return;
     }
     found_last_dot = 0;
     if(*walker == '.')
     {
         found_last_dot = 1; 
     }

     size++;
     walker++;
   }
 

  if(found_last_dot == 1)
  {
   printf("Erro email terminando com .\n\n");
  }

 return;
}

int main(int argc, char *argv[]){

    printf("argv %s\n",argv[1]);

    char *email = NULL;
    int i = 0;
    email = malloc(30);
    strcpy(email,argv[1]);

    printf("Email %s\n", email);
    printf("Email %c\n", *email);

    for(i = 0; *(email+i) != '\0'; i++){  
        printf("Email %c\n", *(email+i));
    }
    int cond;
    cond = caracter_invalido_email(email);
    if(cond == -1)
        return -1;

    consist_email(email);


 
    printf("Email %s\n", email);

    free(email);

    return 1;
}