Login Error

Hi, I had a problem with the login function. The login function is
supposed to authenicate the user email and password and if the both of
them tally with each other, they can then be let into the other pages.
But for my codes, the error is that even if the password and email does
not tally, it can still procceed. Can someone pls help? Thanks! :)

ALTER PROCEDURE spLogin(@MemEmail nvarchar(80), @MemPwd VARCHAR(15))
AS

BEGIN
IF NOT EXISTS
    (SELECT MemEmail
    FROM DasMember
    WHERE MemPwd = @MemPwd)
    RETURN -600

IF NOT EXISTS
    (SELECT MemPwd
    FROM DasMember
    WHERE MemEmail = @MemEmail)
    RETURN -601

END
SELECT MemEmail, MemPwd
FROM DasMember
WHERE (MemPwd = @MemPwd) and (MemEmail = @MemEmail)

IF @@ERROR <> 0
    RETURN @@ERROR

RETURN

DECLARE @status int
EXEC @status = spLogin 'jane@yahoo.com.sg', 'jane'
SELECT 'Status' = @status