bash-4.1$ cat StenKin.txt
 [email protected]
 [email protected]
 [email protected]

bash-4.1$ mapfile -t a < StenKin.txt

bash-4.1$ echo "${a[@]}"
 [email protected]  [email protected]  [email protected]

bash-4.1$ echo "${a[@]//_*@/@}"
 [email protected]  [email protected]  [email protected]

bash-4.1$ (IFS=$'\n'; echo "${a[*]//_*@/@}" ) > StenKin.txt

bash-4.1$ cat StenKin.txt
 [email protected]
 [email protected]
 [email protected]