1. Follow procedure at Configuration/Preferences/Spell Checker/Get more dictionaries... 2. Place new .dic and .aff files in C:\Program Files (x86)\Claws Mail\share\enchant\myspell (I used en_GB-oed from OpenOffice). 3. Follow last line at http://www.claws-mail.org/win32/dictionaries.php i.e. "Restart Claws Mail. Your new dictionary should be available in Tools/Preferences/Spell checking." (Ignoring historical path e.g. Preferences isn't under Tools) 4. Configuration/Preferences/Spell Checker/ Spell checking enabled. But en_GB-oed does not appear in the drop down list. Workaround is to overwrite en_US with en_GB-oed. UPDATE: the problem appears to be the "-" in the en_GB-oed file names. Rename them to en_GBoed and it's all fine!