Why did I have to find out about this? When accepting email invites the default sending account was set to my personal pop account (which has a non standard corporate name). As a result many of my colleagues were complaining who the hell is this bloke!
I've found two options to solve this:
1) force selection of outbound account
This post is stolen from here and i've copied the contents below incase the original disapears.
While we're on the subject of controlling Outlook behavior through policies, I'll mention another registry key that some users might find useful.
In last week's EMO I mentioned a problem some users are having with Outlook's new 'default' account behavior when multiple accounts are delivered to different data files. Outlook uses the email address assigned to the data file you are currently working in as the default for new mail. This is creating problems for IMAP users who have POP3 accounts delivered to the default pst (which is required for IMAP accounts) when they want the IMAP account used as the default for sending. Someone wrote a macro that always uses the default account. Or you can add or edit a registry key to force 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 will also work for people who multiple accounts and frequently accidently send using the wrong account.
Value Name: ForceAccountSelection
Value type: REG_DWORD: 1 for force, 0 to disable.
Then restart outlook to pickup the changes
2) Change account settings
The second option that worked for me was to change the name, email address field and outbound email server. The inbound was left set to the account I really wanted to download from.
The downside of this is you now cannot send any email from this inbound account. However in my case this is a good thing!