sendmail

[Jonathan I. Kamens]

  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 at Athena.MIT.EDU				Allston, MA  02134
Office: 617-253-8085			      Home: 617-782-0710