Here you can find a details explanation and a solution:
http://www.slipstick.com/outlook/outlook-2010/multiple-accounts-and-the-default-account/
(currently down, you can use Google Cache: http://webcache.googleusercontent.com/search?q=cache:2_1XlzFA6qIJ:www.slipstick.com/outlook/outlook-2010/multiple-accounts-and-the-default-account/+&cd=1&hl=de&ct=clnk&gl=de)
a) Registry key in Outlook 2010 SP1 and Outlook 2013
When you have multiple accounts and an IMAP account in Outlook 2010/2013 and you change the default sending account to the IMAP account, you may experience the following issues:
When you click a mailto link from a browser or from Outlook, the default sending account is the Exchange Server account instead of the IMAP account.
When you create a new email message, the Exchange Server account is selected instead of the IMAP account.
Out of the box, Outlook 2010 SP1 and Outlook 2013 use the default account for Send to commands but not for new messages created while viewing a pst file (when a POP account is assigned to the pst file), unless you set a registry value to always force the use of the default account.
To force all new messages to use the default email account, regardless of which pst file you are viewing, browse to the following registry subkey in Outlook 2010 and add a DWORD named NewItemsUseDefaultSendingAccount:
HKEY_CURRENT_USER\Software\Microsoft\Office\14.0\Outlook\Options\Mail
DWORD value: NewItemsUseDefaultSendingAccount
Value: 1
In Outlook 2013, the key is:
HKEY_CURRENT_USER\Software\Microsoft\Office\15.0\Outlook\Options\Mail
DWORD value: NewItemsUseDefaultSendingAccount
Value: 1
b) Registry key to force account selection every time
This fix is simple, but can get annoying, fast, since it forces the user to select an account every time they send a new message. (Replies and Forwards always use the account the message arrived on.)
This might be good for people who use multiple accounts and frequently (accidentally) send using the wrong account, especially if they are sending sensitive documents, as it will almost eliminate accidentally sending email from the wrong account.
HKEY_CURRENT_USER\Software\Policies\Microsoft\Office\14.0\outlook\options
Value Name: ForceAccountSelection
Value type: REG_DWORD: 1 for force, 0 to disable.
In Outlook 2013, the registry key is:
HKEY_CURRENT_USER\Software\Policies\Microsoft\Office\15.0\outlook\options
Value Name: ForceAccountSelection
Value type: REG_DWORD: 1 for force, 0 to disable.
Notes: You need to create the path if it does not exist. You can also set this using the Office 2010 group policy templates.
The value type is the same for both 32-bit and 64-bit Outlook.
This is when you click to create new mail? Perhaps an explicit example scenario and some screen shots would help. The default account feature appears to work for me so I suspect I don't understand your question very well. – Nicole Hamilton – 2013-02-07T17:17:42.707
Sorry, but I don't really know how to make the question clearer (and I wish I could find out what "Ramhound" actually edited above to be sure context wasn't removed). I have an account setup as the default. Outlook does not default to that account as the account that new emails are sent from when creating new emails. I still have to manually select the appropriate account from the "From" dropdown. That's about it. – Boydski – 2013-02-07T18:31:51.597
You can click on the link next to edited (X mins ago) and see the list of changes. He removed the last sentence which wasn't needed to ask your question. – Brad Patton – 2013-02-07T19:08:16.670
I think that comment is rather appropriate. – IAmAN00B – 2013-02-14T15:12:18.873