Wildcards are supported, but don't default host-name, otherwise you might
get unintended results.  Thus, you would probably want to use:

* Filter= Also,arkan*@*

AFAIK, spaces are not a valid syntactic element of the wildcard parser,
though the use of non-contiguous asterisks are e.g.,

* Filter= Also,*postmast*@*

would "catch" any userid that had POSTMAST anywhere within the string:
beginning, end, or anywhere in-between.

When asking about overhead in filtering, don't forget to factor in the
overhead (result) of NOT filtering.

/Pete