Email messages are stuck in Outbox in Outlook 2016 for Mac

Original KB number:  4484206


When you try to send email messages in Outlook 2016 for Mac, the email messages remain stuck in the Outbox folder, and you receive the following error message:

Unexpected data was encountered.


This issue occurs because Outlook 2016 for Mac can't locate and use the Sent Items folder. The folder hierarchy in the mailbox may have become corrupted.

For example, in some cases, the Sent Items folder is converted into the Archive folder, and the icons that mark the two folders are incorrect. See the following screenshots:

A working account A non-working account
correct Sent and Archive folders in a working account
incorrect Sent and Archive folders in a non-working account


To resolve this issue, reset or rename the Outlook folders by using Outlook for Windows.

Method 1: Use the /resetfolders switch

Use the /resetfolders switch to restore the missing folders at the default delivery location. To do this, follow these steps:

  1. Exit Outlook.

  2. Open the Run dialog box:

    • In Windows 10, Windows 8.1, and Windows 8, press Windows logo key +R.
    • In Windows 7 and Windows Vista, select Start > All Programs > Accessories > Run.
  3. In the Run dialog box, enter the following command, and then click OK:

    Outlook.exe /resetfolders

Method 2: Use Outlook Web App

Use Outlook Web App to reset the default folder names. To do this, follow these steps:

  1. Exit Outlook.
  2. Sign in to Outlook Web App by using your credentials.
  3. Select Settings (the gear icon on the right) > Options > General (expand the list in the navigation pane) > Region and time zone.
  4. On the Regional and time zone settings page, change the language, select the date and time format that you want to use, select the Rename default folders so their names match the specified language option, and then select Save.
  5. Exit Outlook Web App.


    If your preferred language is already selected, you may have to select any other language, save the setting, reset the setting to your preferred language, and then save the setting again.

  6. Restart Outlook.

Method 3: Use the Set-MailboxRegionalConfiguration cmdlet

If you're an administrator, run the following cmdlet to reset the default folder names for the user:

set-MailboxRegionalConfiguration -id <alias> -LocalizeDefaultFolderName:$true -Language <Language_code_to_switch_to> -DateFormat <your_preferred_DateFormat>

For more information about language codes and the Set-MailboxRegionalConfiguration cmdlet, see the following websites:

Language codes


Method 4: Use Outlook to select a new Archive folder

To do this, follow these steps:

  1. Download and install MFCMAPI.


    The x86-based version of MFCMAPI should be used with the x86-based version of Office, and the x64-based version of MFCMAPI should be used with the x64-based version of Office.

  2. Right-click the mailbox name at the top of the folder hierarchy in Outlook, select New Folder, and then create a folder that's named Archive01.
  3. Start MFCMAPI in Online mode. To make sure that MFCMAPI is in Online mode, select Tools > Options, and then make sure that the Use the MDB_ONLINE flag and Use the MAPI_NO_CACHE check boxes are selected.
  4. Expand Root Container > Top of Information Store.
  5. Copy the default input ID from the Inbox folder (the renamed Archive folder in this case). To do this, click the Archive folder, select the InternalSchema.ArchiveFolderEntryId (0x35ffXXXX) tag, right-click this tag, select Open Entry ID, and then copy the whole ID.
  6. Select the Archive01 folder that you created, and then examine the item. The following details are displayed:
    • Tag: 0x0FFF0102
    • Type: PT_BINARY
    • Name (s) of property: PR_ENTRYID, PR_MEMBER_ENTRYID, PidTagEntryId, PidTagMemberEntryId, ptagEntryId
    • DASL:
  7. Delete the input ID, and then paste the ID that you copied in step 5.
  8. Close MFCMAPI.
  9. Run Outlook.exe /resetfolders.
  10. Run Outlook.exe /resetfoldernames.

For more information, see Folder names are incorrect or displayed in an incorrect language in Outlook.

Third-party information disclaimer

The third-party products that this article discusses are manufactured by companies that are independent of Microsoft. Microsoft makes no warranty, implied or otherwise, about the performance or reliability of these products.