If you have a certificate error for an email server, your IPOD/IPAD/IPHONE may not notify you of an issue, which doesn't allow you to click OK, which doesn't all you to get email. It simply says, "Cannot connect to server. Check the settings and try again."
find your mail server in the Name column. There may well be more than 1 entry. Find the entry that has “certificate” in the Kind column, and that has the most recent Expires date. This is the certificate you want.
You should now find that you can send and receive email. If not, reboot your IPAD/IPHONE/IPOD.
To find the installed certificate, just tap Settings -> General -> Profile.