Email confirmation link not in confirmation email?

I'm using Elgg 1.7.8, and my host is Arvixe. I recently got an email from someone who tried to sign up on my website. They said that they received the confirmation email, but it did not contain the confirmation link. I wasn't able to reproduce the error. This is the only complaint I've had about this, but I just want to make sure it doesn't happen again. Does anyone have any idea what could have caused this?