How to display char array?

char *str[260];

Is an array of 260 pointers to char, surely you want a 260 chars array:

char str[260];

Furthermore you are returning a local variable that will go out of scope as the function returns leaving a dangling pointer e_mail in main.

An option would be to pass e_mail as an argument of the function:

void email(char *nam, char *ln, char *dep, char *comp, char *str) //str will point to e_mail
{
    strcpy(str, nam);
    strcat(str, ln);
    strcat(str, "@");
    strcat(str, dep);
    strcat(str, ".");
    strcat(str, comp);
    strcat(str, ".com");
}

int main()
{
    char e_mail[260];
    //...
    email(nam, ln, dep, comp, e_mail);
    cout << "email: " << e_mail << "\n has length: " << strlen(e_mail) << "\n";
    return 0;
}

Of course, being C++, you can use std::string instead of C style char arrays.

std::string email(const char *nam, const char *ln, const char *dep, const char *comp)
{
    std::string str;
    str.append(nam).append(ln).append("@").append(dep)
       .append(".").append(comp).append(".com");
    return str;
}

int main()
{
    std::string e_mail;
    //...
    e_mail = email(nam, ln, dep, comp);
    cout << "email: " << e_mail << " has length: " << e_mail.size() << "\n";
    return 0;
}

The problem is that you are returning a pointer to an object that does not live when the function returns.

You can solve the problem using various methods. The simplest one is to return a std::string from the function.

std::string email(char* nam, char* ln,char* dep,char* comp)
{
  char str[260];

  ...

  return std::string(str); // Just to be explicit.
}

and change main to:

int main()
{
    cout << "Hello world!" << endl;
    char nam[50], ln[80], comp[100], dep[30];

    ...

   std::string e_mail = email(nam,ln,dep,comp);
   cout << "email: " << e_mail << "\n has length: " << e_mail.size() << "\n";
   return 0;
}

Ok I figured it out with (https://stackoverflow.com/users/9254539/bessiethecow) help Now it should be just like this:

#include <iostream>
#include<cstring>
#include <stdio.h>
#include <string.h>
using namespace std;
char* email(char* nam, char* ln,char* dep,char* comp)
{
  char*str =new char [260];
  strcpy (str,nam);
  strcat (str,ln);
  strcat (str,"@");
  strcat (str,dep);
  strcat (str,".");
  strcat (str,comp);
  strcat (str,".com");
  return str;
}
int main()
{
    cout << "Hello world!" << endl;
    char nam[50], ln[80], comp[100], dep[30];
char* e_mail = 0;
cout << "Name: ";
cin >> nam;
cout << "Last name: ";
cin >> ln;
cout << "company: ";
cin >> comp;
cout << "Which department: ";
cin >> dep;
e_mail = email(nam,ln,dep,comp);
cout << "email: " << e_mail << "\n has length: " << strlen(e_mail) << "\n";
    return 0;
}