Outlook 2010: Always send from default account (Multiple Mailboxes in Outlook)

* Please make sure you have applied at least SP1

Open Registry.exe and at below path look for a Key called "Mail"

Path: HKEY_CURRENT_USER\Software\Microsoft\Office\14.0\Outlook\Options\Mail

If Key does not exist please right click and create a new Key. Type: Mail and click ok.

Then right click and create a new DWORD (32-bit) Value. Rename it to:
NewItemsUseDefaultSendingAccount

Now open it and type 1 instead of the 0.

Value:

0 = Pre SP1, Uses as sender the email of the mailbox that you are currently in

1 = Uses ONLY the main/default account as sender in all mailboxes

* Credit goes to Microsoft for above.