There's nothing under settings called VPN.
settings > general > VPN (its between cellular and itunes wifi sync on my iphone 5)
There's nothing under settings called VPN.
OP,
I am not sure if you are purposely dancing around this issue, but some yes and no answers seem like they would be helpful. You've been asked, but never really responded...
Are you using an exchange account?
Are you getting this message at home or at work (or both)?
Just reiterating the queations others, who seem to know more than I do about this, are asking. "What do I do to fix this" is hard to answer if you aren't explaining your situation in some detail.
It likely has to do with your Internet connection. Where are you when this pops up? Only on wifi or on cell data too.
If it's an option when you scroll down, accept the certificate.
http://forums.imore.com/iphone-4/224306-what-error.html
Equifax is a SSL Certificate source. Could also be an error on the other end. Most likely, though, an issue between your iPhone and Yahoo Mail's certificate. Might go away on its own in a few days too.
I'm pretty sure I was on Wifi both times.