mh-letter-fill-column

mh-letter-fill-column is a variable defined in `mh-e.el'.
Its value is

72

Documentation:
*Fill column to use in MH Letter mode.

By default, this option is 72 to allow others to quote your
message without line wrapping.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 6.0 of the MH-E package that is part of Emacs 22.1.