Thanks to everyone who weighed in on this. My experience was it is, in fact, the Outlook default data file. To change this I right clicked on the account in the left hand bar >Account Properties > Account Settings > Manage Profiles > Data Files and then I could change the default data file and the correct email is autofilled.
This is a shame as I have 2 companies and will have to make a manual switch for emailing from Quickbooks.
