mh-compose-space-does-completion-flag

mh-compose-space-does-completion-flag is a variable defined in `mh-e.el'.
Its value is

nil

Documentation:
*Non-nil means SPC does completion in message header.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 7.4 of the MH-E package that is part of Emacs 22.1.