Hi Everyone We have 2 seperate Exchange environments here. CompanyA uses a Exchange2003 server along with their own AD domain and Domain controller. CompanyB uses a seperate (logically and physically) Exchange2003 server, AD and Domain controller. CompanyB used to run their email services off CompanyA's servers but have now been migrated to their own servers. Company B can send and receive external email successfully as well as send stuff to CompanyA. My issue is that CompanyA cannot send anything to CompanyB without it returning a 5.2.1 NDR. I've stripped CompanyA's DNS of any mention of CompanyB. There are no receipiant policies that mention CompanyB. 5.2.1 is either Size or permission errors but if CompanyB can accept email from External sources but only fails when it receives email from CompanyA then there must be an issue with CompanyA's server - but where???? many thanks for your time looking at this.