There's also I2P mail, which has a gateway out to the internet at large.
The only problem with I2P mail is that the admins don't want heavy usage, i.e you're only supposed to download mail a few times per day.
That makes sense when it's necessary to operate it as a free service, but it's no longer impossible to make anonymous online payments to services like that.
Perhaps whomever runs that site could be persuaded to make a paid premium version.