The easiest way is just to use wanadoos webmail service from their front page, saves you trying to set it up in your mail client. Your account would still expire after 3 months but all you need to do to prevent that from happening is long on to wanadoo/freeserve at least once in that time using normal dial up.
if using a mail client such as outlook (yuck!
)....
The sending mails bit is irrelevant - you can use btinternets smtp server for any mails you send, no need to worry if you can use freeserves server.
Receiving mails is different, your mail client needs to know what server to get it from. If you are told that something is going to"change your internet settings" its because you are being sent an .ins file that contains instructions for your browser/mail client etc.. you can save this to the desktop instead and read it with notepad and manually add extra info to outllook to stop it overwriting what you already use...but.... Why bother though when you can just use webmail?
you'll find all the settings
here - scroll down to "dial up" bit