ODBEditorという仕組みにExpressoが対応していないのが原因らしい。Text Mate やText Wranglerは対応しているらしい。TextWranglerで試してみてみよう。

Editing with Espresso
dennistoun July 2012
When editing with Espresso, the post I am working on does not update after saving the file and closing the window. Text Mate and Test Wrangler work just fine, but I prefer Espresso.

Anybody else had this problem?
Daniel Jalkut July 2012
Accepted Answer
Hi there – it’s a point I should make clearer in MarsEdit, that ONLY apps that support a specific “external editor” protocol called ODBEditor will work well with MarsEdit’s “Edit With” feature. I am guessing that Espresso doesn’t support this ODBEditor feature, so it doesn’t work well with MarsEdit.

It would be nice to support whatever app regardless, but the system and other apps are not predictable enough in their behavior to make this possible without a clear protocol for communication like ODBEdtior provides.
dennistoun July 2012
Thank you, Daniel. I contacted MacRabbit, the writer of Espresso, requesting ODBEditor in the next version.