Configuring preferences and options in Outlook.com
Am having a little trouble following your question so let me summarize things based on what I understand along with comments just make sure that we are both on the same page. For the sake of this reply am assuming that only one email account is involved. After that will address your issue regarding messages not sending
#1 - You are using Outlook '2010
#2 - Your account is configured as an IMAP account. If this is the case, you will have two PST data files - one for your email folders and the other for the default contact/calendar folders. Each of these PST files will have all the standard Outlook folders (i.e. <Inbox>, <Sent Items> etc). These standard folders will serve no purpose in the <default data file>
#3 - You cannot change the <default data file>.
#4 - The <Sent Items> folder that applies when you send messages is the one in the PST file for your email folders or in other words, the one that is NOT the <default data file>. The default data file will have a check mark beside it
Now to your question/issue
>>
when I try to send email it tries & fails to send from the data file that is default & actually opens in outbox in the other data file but will not send therefore messages just stay in outbox,
<<
As mentioned, the <Outbox> and <Sent Items> folders located in the <default data file> have and serve no purpose. All standard Outlook folders get created by default any time a PST file is set as the <Default Data File>
Am not sure which <Outbox> folder you are referring to - there should be nothing populating the <Outbox> in the <default data file>. If there is then there is something going on that has not been described since that is not the way IMAP accounts work.
Without knowing anything else, in the interests of time, to try and clear this up - would suggest doing the following
#5 - Set Outlook to work in <Offline> mode. Click on the Send/Receive tab and click on it ONCE and wait for Outlook to switch to Offline mode (this is an toggle switch so there is no value in clicking on it multiple times and will only cause a problem.
#6 - When it's in <Offline> mode, close and restart Outlook. Go to the Outbox folder with the stuck items and delete the items.
#7 - Set Outlook to work <Online> (clicking on <Work Offline> once again). Wait for it to return to Online mode. To be on the safe side, close and restart Outlook.
Send a message to see if the problem continues to occur. For the initial test, don't add any attachments, especially large ones, just to keep this as clean as possible.