Unable to set up email on iPhone 4

[#1]

Hello Folks,
I'm trying to set up my mum's email account on her iPhone 4 but without much luck...
Her address is a second email address on my dad's account. I can set his email up on the phone but, with my mum's email address entered, it still sets up his account on the phone.
I have tried changing her email password so that they have different passwords but this doesn't work (although the email client they use on their PC is quite happy with the separate passwords).
Is it possible to set up anything other than the primary email address for a Plusnet account on an iPhone? Any help would be greatly appreciated!
Thanks
Jim

[#2]

A little bit more research and it turns out I was entering the username incorrectly for the second email address.
It needs to be 'account username+mailbox name'... If you created a second email address of 'robert@smith.plus.com' on an account with the user name 'smith', the user name for that new address is 'smith+robert'.