Windows Linux Tips: Sorry but this host is not in my list

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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 

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

"Windows Linux Tips: Sorry but this host is not in my list"

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