I have terminated by email before.

I did it because the family was a constant problem with late payments and they would always pay through email transfer. I knew they'd be a flight risk if I terminated while payment was still due.

So, I waited until they paid one weekend (late as always) and I sent the email after payment was received. I know it sounds completely un professional, but I had to look out for my business first.

The problem with email is knowing if they've received it or not. My client didn't respond at first. So, I had to make the choice to send it again with a text as well to ensure they received it as I didn't want them showing up on Monday not knowing. How awkward would that be!

I also put in my termination that I was giving 2 weeks notice. If they wished to use the 2 weeks they needed to pre pay it.