Most likely dns has not propagated. Wait 24 hours.
if the host send back "domain of mail is not of hdomain of hostname" its 99,99% the responce from certificat.
if dns is not updated, if he setted up correctly the hostname / mailserver (in mailserver manager) , he will not get this message.
if dns is the problem = the hostname will not be resolve = another error message apers in iphone like "Server under xyz.com not found"
i i recreate the scenario, if i insert in my emailsettings from my iphone just the domain, instead fullhostname, i will got the same error like TA
- sry for my bad english, not my native language ^^