MS exchang server error

Since one week ago I begon to get the following erreor in every one (minute) exactley in the event vieuwer (Application logon on the server) The sorce:MSExchange IMC ,the catagory:SMTP interace Events 4106
By Event detail discription I found :Thedialup conniction (… with the ISP…)could not be made,the error reportyed was :[5] Access is denied.
Wht strang is that I have contact with ISP and i can recieve and send E mail through the exchang server .PLZ what is the meaning of this ?nd howcan i stop this error,
thanx for evrybody