CREATE TRIGGER checks_for_validity
	BEFORE INSERT OR UPDATE
	ON employees
	FOR EACH ROW
BEGIN
	IF :NEW.'email' NOT LIKE '%_@%_.__%' THEN
raise_application_error (20501 ,  '`email` column is not valid');
	END IF;
END;

	IF :NEW.email NOT LIKE '%_@%_.__%' THEN

SQL> create table employees(email varchar2(100));

Table EMPLOYEES created.

SQL> CREATE TRIGGER checks_for_validity
  2  	BEFORE INSERT OR UPDATE
  3  	ON employees
  4  	FOR EACH ROW
  5  BEGIN
  6  	IF :NEW.email NOT LIKE '%_@%_.__%' THEN
  7  raise_application_error (20501 ,  '`email` column is not valid');
  8  	END IF;
  9  END;
 10  /

Trigger CHECKS_FOR_VALIDITY compiled

SQL> SHOW ERRORS TRIGGER checks_for_validity;

SQL> COLUMN object_name FORMAT A20;

SQL> SELECT object_name, object_type, status 
  2  FROM user_objects
  3  WHERE object_name = 'CHECKS_FOR_VALIDITY'
  4    AND object_type = 'TRIGGER';

OBJECT_NAME          OBJECT_TYPE             STATUS 
-------------------- ----------------------- -------
CHECKS_FOR_VALIDITY  TRIGGER                 VALID