I have "Text Filter" under "Server Options" set to "Markdown" in the "Compose" window.
In the settings for the blog, I have "Preview Defaults - Preview Text Filter" set to "Markdown" and I have "Post Editor - Server Text Filter" set to Markdown.
When I send the post to Tumble, it ends up as Markdown, but MarsEdit shows it in HTML.
Hi - generally speaking this should not happen, but is it possible you have MarsEdit set to default to Rich Text editing? if you ever switch into the rich text editor, it will convert any Markdown in your post (assuming it's set to Markdown mode) into regular HTML so it can be edited in the rich editor.