Hey We are having a similar problem with Exchange Message flow and this is our first time posting. We need help with this error posted below. The user can send to Mailbox's both internally and externally. The recipient is a distribution list in our other EMAIL application which is MDaemon. And the user can send to any and all of our other mailing lists just fine. No changes have been made to the users Active Directory or Exchange accounts. The error number #550 comes up as SMTP issues with earlier versions of Exchange Diagnostic information for administrators: Generating server: HQ-EXCH1.CBSINC.COM list@email.comcbs-posi.com #550 Sorry, @email.com is not allowed to send mail to a@email.com ## Original message headers: Received: from HQ-EXCH1.CBSINC.COM ([10.0.0.174]) by hq-exch1 ([10.0.0.174])with mapi; Mon, 15 Dec 2008 13:40:53 -0800From: Tracy Davis <td@email.com>To: "a@email.com" <a@email.com>Date: Mon, 15 Dec 2008 13:40:51 -0800Subject: TestThread-Topic: TestThread-Index: Acle/czumRO8aRSER1GUtru4HjE3gw==Message-ID: <497B108708CE0640AF7A82653CC2063F0C5A251968@hq-exch1>Accept-Language: en-USContent-Language: en-USX-MS-Has-Attach: yesX-MS-TNEF-Correlator:acceptlanguage: en-USContent-Type: multipart/related; boundary="_004_497B108708CE0640AF7A82653CC2063F0C5A251968hqexch1_"; type="multipart/alternative"MIME-Version: 1.0

Hi, A user whose mailbox is on Exchange Server failed to send email to a distribution list on another mail server MDaemon. If I am off base, please let me know. I suggest you review the message tracking log on Exchange server to check whether the Exchange server has attempted to deliver the message to the MDaemon. If Exchange server has attempted to deliver the message to MDaemon but failed, I think you need to focus on the MDaemon side instead of the Exchange end. Mike