You could set up a gmail account via tor. Then point a stunnel at
smtp.gmail.com port 465 over Tor using tsocks or something. Making sure
you have a copy of their public cert first and that the stunnel
validates it. I set this up and pointed my MTA (Exim) at it just for a
play at one point. I had to make sure Exim stripped the Received headers
and sent a suitably anonymous EHLO when talking to their service but
otherwise it worked.
It's hard to tell if your SMTP client or stunnel itself send
identifying information. :(