Exchange 2003 undeliverable email for employees that left the company

Have you purged the mail boxes in the system manager?

Do they show up when you click "preview"?
They were deleted, not disabled, correct?

It was purged and it does not appear in Preview. This is also happened when you a send a meeting request to a manager and that his assistant is not working in the company. you get an undeliverable for the assistant email address

IF you go to ADUC do you see the users listed in the member list of the DL?

do you have anything in the company that could be caching the email lists?  for example, some anti-spam servers cache ldap information.

Is the AD account Deleted or Disabled?
If the account is not there can you check with all the user that are member of that group and see if anyone has delegate or forwads to that account.
As if i have a DL and have 10 members in that DL and if there is user5 who had test7 as (delegate or forwad mails to) and if that account is deleted still a NRD would come because the end result is that in the chain the mail could not be delivered to test7.

Thanks for the delegate tip. This is useful. But how about the ones that are not delegate. I went to ADSIedit but I cannot find that user anymore. I tought that Exchange information are stored in AD

You won't find the user in AD at all. But, Outlook caches the delegate information and most of the time doesn't update with exchange. Follow my article and you will stop getting all bounce backs when meetings are requested.

@Rancy: The user is not disabled but has been deleted. I will do what suggested and let you know

Have you tried the solution in my article, regarding meeting requests issue?

Thanks To Rajith and Rancy for their help