Exchange 5.5 I nternet Mail Service fails to start

Hi!
I have exchange 5.5 and 2003 working in one site.
Recently 5.5's internet mail service ceased starting...cannot start it at
all. It gives the following log messages:

The Microsoft Exchange Internet Mail Service, version 5.5.2657.72, is
starting.

Event Type: Error
Event Source: MSExchangeIMC
Event Category: Initialization/Termination
Event ID: 4072
Date:  08-Apr-04
Time:  10:15:10
User:  N/A
Computer: ELLEN
Description:

The Internet Mail Service administrator account is no longer valid. The
Internet Mail Service will not start until you recreate the administrator
account or configure the Internet Mail Service to use another administrator
account.

Internet Mail Service starts with account that is assigned service adiministrator rights, domain admin rights, and that is `service account` in configuration properties.

Any ideas why can this happen and how to resolve this?
Thank you