Xref: utzoo comp.unix.wizards:24496 comp.mail.sendmail:2893 Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!sdd.hp.com!spool.mu.edu!snorkelwacker.mit.edu!bloom-picayune.mit.edu!athena.mit.edu!jik From: jik@athena.mit.edu (Jonathan I. Kamens) Newsgroups: comp.unix.wizards,comp.mail.sendmail Subject: Re: sendmail Message-ID: <1991Mar21.201653.2820@athena.mit.edu> Date: 21 Mar 91 20:16:53 GMT References: Sender: news@athena.mit.edu (News system) Followup-To: comp.mail.sendmail Organization: Massachusetts Institute of Technology Lines: 35 Sendmail uses nameserver Mail eXchange (MX) records to deliver mail to sites that are not directly on the Internet (or that are on the Internet but have requested MX service). The nameserver protocol allows clients to ask nameservers for several different types of information. Two of those are address records and MX records. An address record contains an Internet address. When sendmail is delivering mail to a site that has an Internet address, it asks the name service for the host record and then connects to the address it gets back in order to deliver the mail. An MX record contains a preference and a name. The name is the name of a host to which mail should be sent; that host must have an address record registered. The preference is a number; the higher the number, the more the mailer is encouraged to use that MX record. This way, a host can have multiple MX records, some serving as backup, and the ones with the higher preference will get tried first. The way sendmail actually delivers mail is to *first* ask the name service for an MX record. If it gets one or more back, then the mail is delivered as the response indicates. This allows sites that are on the Internet but that do not accept mail to use the name service to get their mail sent somewhere else. Then, if the MX record query fails, it asks for an address record, and if it gets one back, it tries all of the addresses in the record (hosts can have multiple address records just as they can have multiple MX records, although address records do not have preferences) until it can connect to one. -- Jonathan Kamens USnail: MIT Project Athena 11 Ashford Terrace jik@Athena.MIT.EDU Allston, MA 02134 Office: 617-253-8085 Home: 617-782-0710