If the email program of the sender is broken or misconfigured, plain text emails might not include information about the character encoding used. If you receive such messages, choose ViewCharacter Encoding from the main menu and change the currently chosen character encoding to an appropriate one that might be the encoding used by the sender. You have to make this change every time you view the message.

To make this the default setting, go to EditPreferencesMail PreferencesGeneralMessage DisplayDefault character encoding.