621Chapter 24Running a Mail ServerOnce you have your (Web hosting services)
621Chapter 24Running a Mail ServerOnce you have your A record, you can direct mail to your server using an MX (MaileXchanger) record. The domain for the A and MX records do not need to be part ofthe same domain, which allows for much greater flexibility. Here is a sample MXrecord: widgets.test. IN MX 0 bigserver.example.org. This MX record indicates that mail for (any address)@widgets.test should be sentthrough the server bigserver.example.org. The 0indicates the numeric priorityforthis MX record. When more than one MX record exists for a given label, the MXwith the lowest priority is tried first. If a temporary error is encountered, the nexthighest priority mail server is tried, and so on until the list is exhausted. At thatpoint the sending server will keep trying periodically until the message times out(generally five days). If multiple MX records exist with the same priority, they aretried in a random order. Most mail servers will also fall back on the IP address listed in the A record for alabel in the event that no MX records exist. However, it is considered bad practiceto rely on this. In some cases, it may be complicated to establish an A record because your IPaddress frequently changes. Obviously, this is not suitable for commercial purposes, but there is a workaround that is acceptably reliable for personal use. This is achievedthrough dynamic DNS services that are available (often at no charge) through a num- ber of different companies. A list of these companies is maintained at http:// dmoz.org/Computers/Software/Internet/Servers/Address_Management/ Dynamic_DNS_Services/. Three of the most popular (in no particular order) are: .DynDNS.org (http://dyndns.org/), supported by the ddclient or ipcheckpackages .ZoneEdit (http://zonedit.com/), supported by the ez-ipupdate package .No-IP (http://no-ip.com/), supported by the no-ip packageThe ez-ipupdate package supports all three of these, plus a number of others. View the package description (apt-cache search ez-ipupdate) for moreinformation. Most of these services will provide you with a hostname under a shared domain atno charge and can also provide a similar service for your own domain for a reason- able fee. Configuring for Retrieval from a Mail HostThe configuration requirements when retrieving mail from a mail host are prettylimited. Your server should be ready to accept mail addressed to localhost andshould generally have a name that is unique to it. In the event that a message sentto one of your mailboxes is rejected, the server will need to have a valid host nameby which to identify itself when sending out the DSN (Delivery Status Notification). NoteNote33_
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