Unlike Safari, it’s quite strange and no way to set default encoding in preference in Mail.
Also you may get complaints from friends that using web mail which doesn’t not convert encoding automatically.
To make sure every time when you compose a new email, it’s using the encoding you want, you can invoke this command from Terminal.
e.g. Hong Kong Chinese Character Set
defaults write com.apple.mail NSPreferredMailCharset “BIG-HKSCS”
Apple - Support - Discussions - kmail: Fixing Tiger Mail Encodings
