This message comes up quite frequently for us, but quite often there are different causes. Could anyone list all the known reasons this might come up? All the docs I've found when searching for this relate to problems communicating with external recipients, but I've seen this issue for internal recipients as well. Would be useful to have a list of the causes of this for troubleshooting purposes.

For internal recipient, it basically comes up when some kind of restriction set on the recipient to accept message only from or reject message from. Take an example when you receive this kind of issue next time and verify the Delivery Restrictions are not set to anything... Recipient Properties > Mail Flow Settings Tab > Message Delivery Restrictions http://technet.microsoft.com/en-us/library/bb629676.aspx Amit Tank | MVP Exchange Server | MCITP: EMA | MCSA: M | http://ExchangeShare.WordPress.com

Also please make sure that by any case FROM field is not enabled and you are typing the recipients in From Field. I have seen this issue several timesVinod |CCNA|MCSE 2003 +Messaging|MCTS|ITIL V3|

Hi,For the external recipient, we need to check whether the email is blocked by the security policy on the destinnation server, such as the sender filter, the firewall or permission issue. Additionally, the restriction on the Sender Connector of your Exchange server also can cause this issue.For the internal recipient, you need to check the Message Delivery Restrictions as Amit suggested.ThanksAllen