Why can't I send email from my iPad and phone?

[#1]

We have  been with plusnet for a while,  since our previous broadband supplier sold us to them without notifying us until it had all gone through.
We have surfree.co.uk accounts which work fine on a laptop and iPad when wifi is present at home, however with 3 g or any other wifi we are unable to send emails from the surfree accounts.
The error comes back rejected by server.

We can get incoming email on the surfree addresses but we're a bit fed up of replying to surfree emails from another email supplier!
Any ideas why this happens and if so how to we do it?!
Thanks in advance  

[#2]

You need to setup the username and password for the Sending (SMTP) server to be the same as the details you use for incoming mail.  Authentication is not required (but doesn't cause a problem if it is configured) when you're on a PN network connection but is required when you're on a 3rd party network.  
Details are here and note the section that says

Quote

Outgoing Mail Server settings Host Name: relay.plus.net User Name & Password: These are optional, but if you want to send mail while on the move (e.g. while connected to mobile or wireless networks), we recommend filling these in. Enter the username and password for the email address you're setting up

I've never understood the advice in bold as "we recommend filling these in" is not a good way of saying "make sure you fill these in, otherwise it won't work"!!

[#3]

errors when  I enter the optional username and pass word with the stmp. Relay.plus.net

[#4]

I should have mentioned that you leave the outgoing server as you had it, but add the username and password.
I read your initial post that PN had bought surfree.co.uk and therefore pointed you at the PN setup guides as an example.  I'm really confused now as what you're trying to do and with what address as smtp.surfree.co.uk is an alias for smtp server run by izrsolutions.com and the surfree.co.uk domain is registered through 123-reg.co.uk

[#5]

Quote from: trimom

errors when  I enter the optional username and pass word with the stmp. Relay.plus.net

I believe you need to use smtp.surfree.co.uk as the server address, and your broadband username/alias and password. The username will start with 'adsl'.
If you don't know these details then then it might be worth giving support a call.