It isn't caused by public networks. It's usually caused by a SSL certificate mismatch. This can be caused by the certificate expiring, a problem with the Akamai CDN, or very rarely a man-in-the-middle attack. These types of problems usually resolve themselves within a day or two.
Which email service are you having this error with?
I had this happen on my iPad on iOS5 today. I just shut it off and restarted and no more problem. Only time it ever happened. I was using Apple Mail and Yahoo. I was at home on my own network.