You can’t tag topics this way. Not by ordinary posting, nor by email. We only use the # sign to autocomplete tags, not to add them.
This is understandably confusing to people used to the conventions of mediums like Twitter or Instagram, but makes sense to those more used to GitHub for example. Here’s one previous discussion about this.
We’re not going to change this convention (i.e. we’re sticking with GitHub’s take on hashtags), but I’d welcome ideas on how to better educate users about it.