I have been bit by this more times than I can count. I have Outlook 2010. Two email accounts. The "Default" is a POP3 used to send and receive. The second is an Exchange Server 2003 account used solely for storage. You cannot send or receive on it. It is storage only.
Well... I hit a problem with Adobe Reader yesterday where if you send the attachment from withing Adobe Reader it would bounce as undeliverable. It took me all day to figure it out but when Acrobat opens Outlook 2010 it is not sending on the Default account. For some reason it by default tries to send on the Exchange account and hence the bounce.
This has happend many times before with third party programs trying to send via Outlook. The third party programs won't use the account you specify in Outlook as the Default. How frustrating.
One thread says that Outlook will try and determing the best account to use. If so what is the point of setting the default account? Who is the idiot here? Microsoft or the third party programs like Acrobat Reader? Is there a solution?