Here's a step-by-step guide on how to trouble-shoot Postbox. Please follow each step in sequence.

1) Use the Trouble-Shooting Panel

Postbox 3.0.2 and later contains a new Trouble-Shooting panel to help address issues you may be experiencing with Postbox. Go to the Help menu and select Trouble Shooting..., and then select one or more of these options:

• Safe Mode - This disables all add-ons and custom settings. 
• Permanently Disable all Add-ons - this eliminates the possibility that a 3rd party add-on is causing issues with Postbox.
• Reset User Interface - If Postbox windows are not displaying properly, selecting this option will reset the interface.
• Reset Search Index - This removes the search index, so you will want to select Tools / Indexing / Index All Messages after restart.

3) Create a New Profile

A useful diagnostic step is to see if an issue goes away within a new profile:

1) Quit or exit Postbox so that it is no longer running.

2) Launch the Profile Manager:

Windows
From the Windows "Start" menu, choose "Run" (this is "Start Search" on Windows Vista") then type in the following:
postbox.exe -ProfileManager

Mac OS X
Launch Terminal on the Mac - Applications / Utilities / Terminal and then paste in the following:
/Applications/Postbox.app/Contents/MacOS/postbox-bin -profilemanager

3) Use the Profile Manager to create a new profile. Make sure that the "Don't ask at startup" checkbox is NOT checked.

4) Next, set up a sample IMAP account.  Do NOT install any Add-ons!

If the issue(s) go away with the new profile, then migrate your settings to this new profile and use it moving forward.

If the issues persist, move to the next step.

4) Create a New OS User Account

Sometimes there are issues external to Postbox which are causing it to malfunction.  For example, anti-virus or backup software can prevent Postbox from saving preferences, folder permissions issues on disk can prevent Postbox from being able to save messages, etc.  

The easiest way to trouble-shoot these types of issues is to setup a new OS user account to see if the problems persist.

1) Create a new user account on your Operating System (please refer to your OS user manual for instructions)

2) Download and install Postbox.

3) Next, set up a sample IMAP account. Do NOT install any Add-ons!

If the issues go away with the new OS user account, then you most likely have a disk permissions problem, or a third party application or utility that is interferring with Postbox's operation.