If you would like to prevent the app from starting automatically:
On Windows and Mac: go to Settings | Global Settings | Advanced Features and turn off Launch app on system startup. The setting is offered on those two desktop builds; on Linux, autostart is handled by your desktop environment rather than by BlueMail.
The same screen also offers Enabled, Minimized and Disabled for how the app starts, so you can have it launch minimized rather than not at all.
