In the WA_UniversalEmail folder, you will need to edit each of the files whose names begins with "WAUE_" for example:
WAUE_confirm_1.php
in each of those files, change line 15:
$WA_MailObject = WAUE_Definition("".($WAGLOBAL_Email_Server) ."","25","","","","");
to use the BIG5 character set:
$WA_MailObject = WAUE_Definition("".($WAGLOBAL_Email_Server) ."","25","","","","BIG5");