Sorry but this host is not in my list

Mail clients require you to specify your server for incoming mail, but they also need a server for outgoing mail. If you have sendmail or Qmail running, you can use localhost. If not, you may use your provider's mail server.

If you setup your own mail server, you may have problems sending mail from other systems on your network. Mail servers often block clients from using them for outgoing mail to prevent relaying. You need to specify the hosts where you will be sending mail from. Here is how to do it in Qmail, from the FAQ. You need to put the following line in /etc/hosts.allow:

tcp-env: ip1, ip2, ip3: setenv = RELAYCLIENT