In the 1990s, email was in its infant stages. If "you got mail," you probably would've opened it, no matter who it was from. Today, with all kinds of spam and viruses being dropped on our e-doorstep. we've become a bit more skeptical.
In fact, lots of times people won't even open an email if they are not absolutely certain who it's from. Keep this in mind if you're creating an online survey -- especially when you are sending out the invites.
If the link will be sent by the survey site itself you should make sure everyone knows it's coming. Send a personal email from yourself that announces the survey. This way they won't accidentally banish it to the e-trash.