The Postbox Profile Manager
Postbox has a profile manager which can be used to manage multiple profiles. This shouldn't be confused with the ability to support multiple accounts which you can do within a single profile.
The profile manger allows you to switch between profiles in addition to creating and deleting profiles.
Accessing the Profile Manager Using the Preferences Pane
1) Go to Preferences (macOS) or Options (PC) / General, and select the checkbox for "Show Profile Manager on startup".
2) Restart Postbox.
Accessing the Profile Manager Using the Command Line
Make sure Postbox is not already running.
From the Windows "Start" menu, choose "Run" (this is "Start Search" on Windows Vista") then type in the following:
Launch Terminal on the Mac - Applications / Utilities / Terminal and then type in:
How to Delete a Profile
To delete a profile, right-click it and then select Delete from the popup menu.