If you want to use multiple accounts in Outlook and one is an Exchange server, the Exchange account always wants to be the default data file. While you can set any email account as the default account for sending mail, you may not be able to set the data file as default in the Account Settings dialog.
While this was fixed in Outlook 2016 (when you have only 2 accounts in your profile), if you use an older build (such as is available with the retail version), you will need to use the following steps to change the default if you have a business Microsoft Exchange account.
When you try, you might receive and error message that reads:
The selected Outlook data file (.pst) format cannot be used. Outlook data files (.pst) must use the same format as offline Outlook data files (.ost). Select a matching Outlook data file (.pst) or turn off cache mode in the Account Settings.
However, you can set another data file as default if you add the Exchange account to the profile from the Control Panel's Mail applet and follow specific steps. If the Exchange account was the first account added to the current profile, you'll need to make a new profile.
Note: It doesn't matter which account is added to the profile first but you absolutely must add the Exchange or Outlook.com account to the profile from Control Panel and disable cached mode (Step 2) on the account before closing the dialogs.
If using a Google Apps account, let Google Apps create the profile and open Outlook (at least) once to sync the account. Close Outlook and add the Exchange or Outlook.com account to your profile from the Control Panel.
- Open the Control Panel and search for Mail
- If your profile opens directly when you click the Outlook icon, click Email Accounts. If you are creating a new profile, or have more than one profile, click Show Profiles then add a new profile or select the correct profile.
- Add the Exchange (or Outlook.com) account to the profile.
- Click the Change Account Settings box (Manually Configure settings in older versions) after your account is configured and then click the Next button.
- Uncheck the Use Cached mode option to disable cached mode. If the Use Cached Exchange mode is grayed out, click More Settings, then Advanced and untick Use Cached Exchange Mode.
- Click Data Files and set the desired data file as the default.
- Close the Account Settings dialog to return to Mail setup dialog (screenshot in Step 2) then click Data Files button to re-open Account Settings dialog.
- Verify the default data file is correct.
- If you get the message that you can't set the other data file as default, double-click on the Exchange data file and untick Use Cached Exchange on the Advanced tab again.
- From Email tab, double click on the Exchange account and re-enable cached mode.
- Close the dialogs and return to the Mail applet. Reopen the profile and verify the correct account and data file is set as default.
- If you created a new profile, set it as the default profile.
- Close the dialogs.
- Restart Outlook.