So what do your message tracking and smtp logs say when this happens? Regards, Michael B. Smith MCSE/Exchange MVP http://TheEssentialExchange.com From: exchangelist-bounce@xxxxxxxxxxxxx [mailto:exchangelist-bounce@xxxxxxxxxxxxx] On Behalf Of Arnold, Jamie Sent: Wednesday, December 05, 2007 4:02 PM To: exchangelist@xxxxxxxxxxxxx Subject: [ExchangeList] NDR from good mailbox I have a mailbox that I can send to from my Outlook client, but when I send to the same address via any other method, it often, but not always, sends an NDR. The AD object has what appears to be 2 good SMTP addresses and a valid X400 address. If I send from Gmail (for example) I get this error message: 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. <bux2k.binghamton.edu #5.1.1> This does not happen every time.