The dump subcommand dumps the database to the standard output in a format suitable for backup or sending over the network (a version 1.0 format).
mu dbm dump file.db < file.dump
The produced file is suitable for input to the create (load) command. Among other uses, it provides an easy way to convert databases between various formats supported by Mailutils. For example this is how to convert the database file file.db to the GDBM database new.db:
mu dbm dump file.db | mu dbm create gdbm://new.db
Both list and dump subcommands share the same set of options. In fact, they are pretty similar, except that use different defaults. The list subcommand is designed to produce a human-readable output, whereas the dump subcommand is oriented towards backup purposes.