Check the following lines in your ~/.muttrc if you're migrating from
mutt 1.2.x.

set charset=big5
set locale="zh_TW.Big5"
set send_charset="us-ascii:iso-8859-1:big5:gb2312:utf-8:x-unknown"
charset-hook us-ascii big5
charset-hook iso-8859-1 big5

Please execute the following command if you encounter problem with
legacy bbs:

echo "set bbsislame=yes" >> ~/.muttrc

If you have to send signed mail to outlook users, please execute the
following command:

echo "set pgp_outlook_compat=yes" >> ~/.muttrc