I have two users this is a problem for and have not had much luck finding an answer.Per KB 924416 I checked that there is a trailing semi-colon in the x.400 address. There is. However, if I edit the properties of the x.400 address by adding a space and deleting the space and hitting apply, the problem is resolved for a few hours. I'm running Exchange 2003 sp2. Both these users use Blackberries but the other Blackberry users are not having this problem. Below is the full text of the NDR message Your message did not reach some or all of the intended recipients. Subject: Outlook on your Blackberry Sent: 2/21/2008 1:24 PM The following recipient(s) cannot be reached: 2/21/2008 1:24 PM The e-mail account does not exist at the organization this message was sent to. Check the e-mail address, or contact the recipient directly to find out the correct address. <mail.state.mn.us #5.1.1 smtp;550 5.1.1 User unknown> I hope I have posted this in correct forum and I hope someone can help. Thank you. K