#4 Mozilla Thunderbird does not use UTF-8 as default encoding.

[Sorin Sbarnea (old)]

In order to manually change this you must go do:

View > Character Encoding > Unicode (UTF-8)
Tools > Options > Display > Formatting > Fonts > Character Encodings > Outgoing Mail > Unicode (UTF-8)
Tools > Options > Display > Formatting > Fonts > Character Encodings > Incomming Mail > Unicode (UTF-8)
Tools > Options > Display > Formatting > Fonts > Character Encodings > Check the "Use the default character..."

The application must have these configured as defaults because this does solve most problems for all languages.