Home Web Servers, Part 2: Software and Maintenance - Mail Server Security
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Security for a mail server is also very important. SMTP servers have the capability to relay messages for other servers. Spammers often exploit this capability to send huge amounts of difficult to block spam messages. When you set up an email server, you must be careful to make sure your SMTP relaying is turned off to only allow either authenticated users or local users (behind a firewall or from a specific IP address) to send email through your server. This will not prevent your server from receiving mail from other servers, but it will prevent you from unknowingly taking part in the great scourge of Internet spam.
If you do not make sure your email server is locked down from relaying, you run the risk of having your server placed on a Relay Blacklist. These are lists of open relay SMTP servers; system administrators can then setup their own SMTP servers so that they do not receive email from them. If you server is placed on one of these lists, you run the risk of having your email refused from email servers to which you wish to send. Conversely, after you set up an email server, configuring yours to use a Relay Blacklist is often a great way to reduce spam.
Another service you may want to check on is Reverse DNS. Some email servers will refuse email sent from servers without a Reverse DNS listing. This is a listing that your ISP needs to set up, as they control the listings for all the IP addresses that they own. Often, if your ISP offers you a static IP, they will be willing to do a Reverse DNS listing that links your IP address to your domain name. This is something you will need to check on with them. In general, setting up and running an email server often has more responsibility associated with it than other servers you may wish to run. Make sure you carefully check your settings before taking your server live on the Internet.
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