Since I can't use my favorite blogging Mac app to publish directly to Squarespace and Squarespace post by email is well just a "pos" option. I decided to give Squarespace other option a try.
And that option is installing a blog editor bookmarklet to Safari (or any other browser).
This option is a little better as no matter where you are just pressing the icon on the browser will launch the editor with no need to login into Squarespace.
And unlike the post by email option I can add tags email and images instantly as it works just like the normal web editor on Squarespace.
I still hope Squarespace open it's API so Deve can create awesome apps like Desk and we don't have to relay on this workarounds.