[phubeone]

Where are you getting this error, in outlook, lotus....? Are you running an exchange server, are you using POP, IMAP? You're trying to send mail to a specific department, is that on a distribution list, group you created in outlook?

 

[phubeone]

Create a new distribution list and re-add the users. See if you can send an email to that distribution list, if you can then recreate the engineer distribution list.

Where do you see this error? In outlook or in the event logs on the server.