This article was previously published under Q222329
NOTE: The procedures in this article only apply if you have installed Outlook with the Corporate or Workgroup option. With this option, you can use Messaging Application Programming Interface (MAPI) services. To determine your installation type, click About Microsoft Outlook on the Help menu. If you have the Corporate or Workgroup installation, you see "Corporate or Workgroup".
When you attempt to send e-mail to recipients included in a contact Distribution List (DL), you may receive one of the following errors:
The messaging interface has returned an unknown error. If the problem persists, restart Outlook.
An internal support function returned an error.
There is a duplicate, one-off contact in the DL, containing the Internet address for a member of the Microsoft Exchange Server Global Address List (GAL).
Do not use the identical display name for one-off contacts in a DL that also have Internet entries in the GAL.
If you need to include a GAL member's Internet address in a contact DL, use a different display name.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed at the beginning of this article.
A one-off address entry is made in a DL by clicking the Add New button and then adding the recipient and Internet e-mail address without adding the recipient as a contact.