My guess is that they would have to check their email on their smart phone, but I want to send an email to someone who doesn't use the computer on weekends, and I know she has her phone turned off right now.
She would receive the E-Mail when the phone was turned on probably.
Believe it or not, you can send an email to somebody who doesn't have a smart phone with email capabilities, as long as they can get text messages and as long as the email you end is only text. The wireless providers have email to SMS conversion, so you just send to an email address that starts with their phone number ad ends with the provider's conversion address.
Here's an article with a good description and all the addresses.