If you happen to have an email address that uses an International Domain Name (IDN), i.e. one which includes non-ASCII characters (like info@蓝天东.org), you will find that it's not possible to set up an account in iOS Mail, even when you use the Punycode/ascii version (email@example.com in the case mentioned earlier). It just says the address is invalid and you cannot go further. You can, however, set up such an account in OS X Mail.
One possible iPad work around is to try the alternative free email app Incredimail, which normally lets you set up everything without problem. It does not yet support POP, however, only IMAP. Another work around is to have your IDN at Godaddy, which has its own app for doing email hosted by its servers.