I too got the "The IMAP server, "mx.000webhost.com" is not repsponding. Check your network connection and that you entered the correct infomation in the "incoming mail server" message when all my settings seemed ok.
I tried disabling my wifi as sternedwards suggested, but it still didn't work. Then I tried deleting the account on my iphone completely (I had been editing the existing entry used for a previous host) and creating a new one. This time (after the SSL message to which I answer YES) I got a message saying my username and password were wrong. So that was progress.
I carefully re-entered the username and password, no go, tried my 000webhost username of a123456 and site password, no go, tried my 000webhost login email address and password, no go, and finally went back to my email account username and password, still no go.
The only thing I could think to do was to change my password on the control panel (which was working fine from my PC) and that worked!? Now perhaps I had finger trouble along the way but I was careful. Anyway, for those setting this up it's something to try if all else fails.
For the record, on the iphone my settings are...
IMAP Account information:
Name: Phillip Biggs
Incoming Mail Server:
Host Name: mx.000webhost.com
User Name: email@example.com
Password: the password for that email address
Outgoing Mail Server
No user or password supplied.
All the above was done with wifi turned off. It might have worked with it on, but now I've got it working I'm not going to experiment
Hope that helps people through any confusion.