mh-show-threads-flag

mh-show-threads-flag is a variable defined in `mh-e.el'.
Its value is

nil

Documentation:
*Non-nil means new folders start in threaded mode.

Threading large number of messages can be time consuming so this
option is turned off by default. If you turn this option on, then
threading will be done only if the number of messages being
threaded is less than `mh-large-folder'.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 7.1 of the MH-E package that is part of Emacs 22.1.