The message could not be delivered because the recipient's destination email system is unknown or invalid. Please check the address and try again, or contact your system administrator to verify connectivity to the email system of the recipient. <xxx.xxx.com #5.1.2> you do not have permission to send to this recipient. For assistance, contact your system administrator. MSEXCH:MSExchangeIS:/DC=com/DC=domain:servername these are the two random error some of the users are getting while sending messeges internal or external randomly. well i found out the cause of these errors this could be due to because we recently migrated our exchange server on a new harware due to hard disk crash on old server. so i had to delete the mailboxes and had to recreate it on the new server and restore from the backup. But servers are in the same organisation. 1. i tried to add the x500 address to all the users but the problem is same or may be i am not doing it properly. if this is the solution to this problem can anyone suggest me how to add x500 address like what format. e.g /o=? /ou=? /cn=? /alias=iknow please reply if any other details needed but please do reply regards lovemsexchange