Custom markup to match category?

You can use the postie_post_before filter to parse the subject line and update the post_category element of the $post array.
See https://postieplugin.com/postie_post_before/

@wayneallen-1 Thank you, that worked.

But I had to convert category_name to category_id to make it work, even if wp_insert_post accepts

Array of category names, slugs, or IDs. Defaults to value of the ‘default_category’ option

.