In Safari?
If you're connecting to a webmail site then the message probably has nothing to do with the iPad. You'll want to check and see if your email provider isn't' having problems. Or it might be a network issue, but Johanna has already covered that.
If not, then chances are it's Mail trying to update in the background, and you just happen to have Safari open. Or maybe updating Mail when it detects an active internet connection is part of the new background update feature.
Try opening Mail, forcing an update by pulling down on the mail list, and see if you get the same error. While there are any number of things you can do in your account settings to fix, it's usually easier to just delete and re-add the account. If the account is a POP account, make sure you don't' have any important emails stored only on the iPad, otherwise everything is on the server and should not be a problem.
If you don't know what type of account you have and can't find out from your provider, let us know who your provider is and maybe we can figure it out.