Re: Relaying mail?
It's not so much the addresses attached to an email - for example, the "reply to" address is merely a text label you set up in your email program.
Most ISP mail servers will only accept connections from hosts valid on their own networks - again, for example, your internet provider on your dial up account will have an SMTP server address, that will accept requests on TCP port 25 from hosts within its' network. When you allow hosts *outside* your network to send traffic to your mail server, this is what "relaying" is, as when you "relay" mail, *your* mail server appears as the sender, not the "originating" server.
Almost every ISP on the planet will ban relaying to try and prevent spam. If you really want to use your "ntlworld.com" account at work, you should be able to configure your mail client with the name and account details of the NTL POP server (to be able to receive mail) and then set the SMTP server address as your dial-up ISP's SMTP server.
As long as you set the "Reply-To" field to your @ntlworld.com address, all should be well...? As Robin notes above, this is the *correct* way to do it...
Which dial-up ISP are you using by the way...?