mh-update-sequences-after-mh-show-flag

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

t

Documentation:
*Non-nil means flush MH sequences to disk after message is shown.

Three sequences are maintained internally by MH-E and pushed out
to MH when a message is shown. They include the sequence
specified by your "Unseen-Sequence:" profile entry, "cur",
and the sequence listed by the option `mh-tick-seq' which is
"tick" by default. If you do not like this behavior, turn off
this option. You can then update the state manually with the
x, q, or M-x mh-update-sequences
commands.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 7.0 of the MH-E package that is part of Emacs 22.1.