Preferences Window is Too Narrow

If the Preference window is too narrow and some of the contents are not visible, try this workaround:

1) Quit Postbox

2) Find your profile directory (see for details.)

3) Create a chrome directory under your profile directory.  Something like:

macOS: ~/Library/Application\ Support/Postbox/Profiles/<random>.default/chrome

Vista: C:\Users\<user>\AppData\Roaming\Postbox\Profiles\<random>.default\chrome

4) In the chrome directory, create a plain text file named userChrome.css with the following contents:

 * Do not remove the @namespace line -- it's required for correct functioning
@namespace url("");

#MailPreferences {width: 60em !important;}