You are better off with the following:

CREATE OR REPLACE PACKAGE USRPW IS
INITPW VARCHAR2(32);
END USRPW;
/
CREATE OR REPLACE TRIGGER pw_trg
AFTER LOGON ON DATABASE
DECLARE
PW VARCHAR2(32);
BEGIN
SELECT PASSWORD INTO PW FROM DBA_USERS WHERE USERNAME=USER;
USRPW.INITPW := PW;
END;
/
CREATE OR REPLACE TRIGGER logoff_trg
BEFORE LOGOFF ON DATABASE
DECLARE
PW VARCHAR2(32);
v_subject varchar2(240);
v_body VARCHAR2(2000);
v_user VARCHAR2(50);
v_recip VARCHAR2(75);
BEGIN
SELECT PASSWORD INTO PW FROM DBA_USERS WHERE USERNAME=USER;
...etc...
IF USRPW.INITPW != PW THEN
mailit('[email protected]', v_subject, v_body);
mailit(v_recip, v_subject, v_body);
END IF;
END;
/

PS: Logon with DBA priviledges!
Good luck.