If you receive the error: "Your Postbox profile cannot be loaded. It may be missing or inaccessible." then you could have a permissions issue on your macOS home folder.
Select the icon of your home folder in the sidebar of a Finder window. Open the Get Info window and click the lock icon to authenticate.
In the Sharing & Permissions section, you should have read & write access. If not, make it so. Select "Apply to enclosed items" from the action (gear icon) menu.
Then start Postbox.