Mail send from a second exchange account with "send as" permissions is not stored in the proper sent mail folder

1. To do this, follow these steps:
   1. Click Start, click Run, type regedit, and then click OK.
   2. Locate and then click the following registry subkey:
      HKEY_CURRENT_USER\Software\Microsoft\Office\12.0\Outlook\Preferences
   3. On the Edit menu, point to New, and then click DWORD Value.
   4. Type DelegateSentItemsStyle, and then press ENTER.
   5. Right-click
      DelegateSentItemsStyle
      , and then click Modify.
   6. In the Value data box, type 1, and then click OK.
   7. Exit Registry Editor.

Important After you set the DelegateSentItemsStyle registry value to 1, the functionality is only available when the Microsoft Exchange account is set to Use Cached Exchange Mode. The DelegateSentItemsStyle registry value will not work consistently on an Exchange account that is configured in Online mode. For more information about enabling Use Cached Exchange Mode, see the following Microsoft Office support article:

http://office.microsoft.com/en-us/outlook-help/redir/HP001232935.aspx?queryid=cde1b9af5cc1430a8e0adaafea0c67aa&respos=0&CTT=1

Note If you must use Outlook in Online mode and if your mailbox is located on an Exchange Server 2010 server, the Exchange administrator can configure similar behavior on the server. Update Rollup 4 for Exchange Server 2010 Service Pack 2 introduces a new Exchange PowerShell cmdlet to configure which Sent Items folder a message is copied to. Because this new feature is handled by the Exchange server, Outlook can be configured for either Online or Cached Exchange mode. However, the Exchange server feature only works if the Outlook DelegateSentItemsStyle registry value is disabled.. 

For more information about the Set-MailboxSentItemsConfiguration cmdlet, click the following article number to view the article in the Microsoft Knowledge Base:

2632409 Messages sent by using the "Send As" and "Send on behalf" permissions are only copied to the Sent Items folder of the sender in an Exchange Server 2010 environment