Hi 61600,
If your emails are sent directly from AccountRight, they will be sent from an APP.myob.com email address, which can't be changed. However, you can set your "Reply-To" email address so that recipients can respond to your updated company email. For instructions on how to set this up, see Set up AccountRight to send emails.
But if your emails are sent through Microsoft Outlook, the email address used will be whichever email address is set as the default in Outlook. See Sending emails using Microsoft Outlook for information on setting your default email address.
Regards,
Sai