It's not that hard to set up your email account to send automated emails in the future. Email marketers do it all the time. They will spend a day writing up a bunch of emails, set them to send at certain dates and then they have the rest of the week free to do other things. I can even do it with my blog. I write out the post, set the date that I want it to be posted and if I die between now and that date, people will think that I am "blogging from beyond the grave" if they are gullible enough.
The fact that the email included a personal joke between he and his friend shows that the email was from him and not a hacker, but he would have just written up a bunch of emails while he was alive and set them up to send at certain dates after his death.
I am surprised that there is a service for it though, I hope people don't have to pay for this service. It can be easily done with a gmail account and I think you can do it with hotmail too. I am sure that there are many other free email services that allow you to do the same thing.