Here's how Identity selection works when using the Compose window.
The send Identity that will be selected when you open the Compose window is based on the account that you have selected when you click the Compose button. If you have multiple Identities assigned to that account, Postbox will select the default Identity, which is the first Identity that is created when you set up the account. To switch Identities, use the Identity bar in the Compose window, which is located above the To: addressing field. Within an account, you can also select a different Identity as default, and assign a different send server to that.
If you have an Account Group selected, then the Identity selected in the Compose Window will be from the first account listed within the Account Group, and the default Identity for that account.
For cases when no account is selected, for example, if you click a mailto: link in a browser, then the sending server that is set as default will be used.