Update: This hotfix is included in Outlook 2010 SP1. See Default account fix for more information and the required registry setting.
When clicking on a "mailto:" on a web page or using the Send to command in other applications, Outlook opens a new email. In Outlook 2010, when the default account is an IMAP account, it uses the account assigned to the default message store, not the IMAP account.
Outlook 2010 SP1 also addresses an issue with Read Receipt notifications when you use a VBA script to send messages and are requesting receipts.
MSKB article describing this change, first released as a hotfix: Description of the Outlook 2010 hotfix package (outlook-x-none): May 10, 2011
For more information and other solutions, see Microsoft Outlook 2010, Multiple Accounts and the Default Account
The IMAP 'no new mail envelope icon' hotfix is also included in SP1.
Published June 10, 2011. Last updated on May 8, 2014.