How to Set the Default Account When Launching the App?

You can choose which account or view BlueMail opens to by default.

Go to Settings | Global Settings | Advanced Features | Default Account when Launching App, then choose:

Unified Inbox — always opens the combined view of all your accounts. Tap an email notification and you still land in the Unified Inbox, with that message waiting there.
Unified (excluding notifications) — opens the Unified Inbox when you start the app yourself, and takes you straight into the relevant account when you arrive by tapping a notification. The exception is what "excluding notifications" refers to.
Last Visited Account — reopens whichever account or view you last had open, and likewise jumps to the account of any notification you tap.

Settings → Global Settings → Advanced Features → Default Account when Launching App

Looking for the account BlueMail sends new mail from in the Unified Inbox? That is a separate setting, Default From in Unified Inbox, on the same Advanced Features screen. See Setting the Default 'From' Account in the Unified Inbox.