I recently had this exact question. I finally decided on Pawnmail (https://pawnmail.com/). It's free and you get 2 GB.

I was worried whether it'd randomly shut down, so I emailed the creator, http://pastebin.com/raw/eWG8RQqk, and it looks like it'll be safe for a while.

I have no idea whether using services like these is a good idea.