When you send email messages in Microsoft Outlook, you receive the following error message:
The operation failed. The messaging interfaces have returned an unknown error. If the problem persists, restart Outlook.
This issue occurs when a Google Apps Sync for Microsoft Outlook account and a Microsoft Exchange Server account are configured in the same Outlook profile.
Do not configure an Exchange Server account and a Google Apps Sync for Microsoft Outlook account in the same Outlook profile. Instead, create separate Outlook profiles, and then configure the Google Apps Sync for Microsoft Outlook account in one profile and the Exchange Server account in another profile. The following Microsoft Knowledge Base article describes how to create profile and set up an e-mail account in Outlook.