Consider the following scenarios.
You use Microsoft Outlook 2010 to configure multiple Microsoft Exchange Server mailbox accounts.
The secondary mailbox account has permission to send to a restricted distribution group (DG).
You create a new email message, address it to a restricted DG, and set the secondary Exchange account as the email account from which to send the email message.
Your primary mailbox account does not have permission to send to the restricted DG.
You have SendAs permissions to another Microsoft Exchange Server mailbox.
You connect to your mailbox by using Outlook 2010.
You create a new email message, add the other mailbox's name in the From: field, and address it to the restricted DG.
In both scenarios, the following MailTip is displayed in the new email message:
You don't have permission to send to: distribution group
In this MailTip example, distribution group is a placeholder for the name of the restricted distribution group.
With the MailTips feature, Microsoft Outlook 2010 sends your user account details to Microsoft Exchange Server 2010. Exchange Server 2010 evaluates your mailbox's permissions to access a restricted DG. Exchange does not assess permissions based on the email account to which you have Send As permissions. The result of the permissions check is reflected in the MailTip message. However, if you send the email message from the mailbox account for which you have Send As permissions, the email message is sent and delivered successfully to the distribution group.
Microsoft is researching this problem and will post more information in this article when the information becomes available.