How do I use tags (aka labels) if I post by email (aka Mail-to-Blogger)?and the unfortunate response is
You can't add labels to a post through Mail-to-Blogger.
Mail-to-Blogger lets you post text, with titles. That's it. No adding or editing labels, no changing the post date, no selecting backlinks or comments for the post being created.
With Mail-to-Blogger, you write a post, give it a title, and send it to Blogger. Anything more requires either the "Edit Posts" menu, or "Post Options" in Post Editor.