We have a customer who just merged with another organization. The Exchange 2010 server is setup to receive email for both abc.com and xyx.com representing the two different companies. The users want to be able to send email from both of these email addresses while one is set to the primary. In the past (Exchange 2003), one would simply setup a POP account that would allow the user to select a different account from the drop-down accounts box (FROM) in Outlook and then the user could send the email out. This is still configurable in Outlook 2010 but for some reason we cannot get authentication to succeed. Every time you try to send a message it simply pops up with the authentication dialogue box.

Are you using port 25 or 587 to send? By default, the Receive Connector which is what you are sending through, doesn't support authenticated relaying as a security measure. You would need to configure the clients to use TLS and the alternative port (as configured on the Client Receive Connector). Simon.Simon Butler, Exchange MVP Blog | Exchange Resources

As Simon mentioned, you can create additional Receive Connector and customize the authentication or port. To create Receive Connector, please refer to the link below: http://technet.microsoft.com/en-us/library/bb125159.aspx Thanks. NovakPlease remember to click Mark as Answer on the post that helps you, and to click Unmark as Answer if a marked post does not actually answer your question. This can be beneficial to other community members reading the thread.