Host Database Field: MAIL-METHOD

[This document describes a field in the Princeton University Host Database. You may also view descriptions of other fields.]

This optional field is used to instruct other hosts on the Internet where email addressed to this host should actually be sent; if specified, it causes an MX record to be generated in DNS. (The MX record's label is the host's canonical name, and the record's value is based on the value you specify in this field, possibly modified by the DNS-DOMAIN field.) If MAIL-METHOD is not set, no MX record is generated.

This field is alternatively known as "Mail Exchanger" or simply "MX".

Unless you really know what you're doing, you should not set this field; most hosts do not actually receive mail themselves nor do they have some other host accept mail on their behalf.

If the field is set, and you wish to clear it, you may specify the value NONE.

Given a host with a canonical name of foo, here are some common values for the MAIL-METHOD field:

• If foo is like most hosts, and does not run an SMTP server (i.e. it cannot receive email addressed to user@foo), leave this field blank, which is the default.

  This in turn will cause host foo to receive no MX record.

  If you have mistakenly set this field in the past, and wish to clear it, specify the value NONE.

• If foo runs an SMTP server (it can receive email addressed to user@foo), and you want others to be able address their mail to user@foo, specify "foo".

  This will cause host foo to receive an MX record pointing to "foo.Princeton.EDU".

  Note that the name you enter for the MAIL-METHOD should not usually end with a dot; the resulting MX record will have ".Princeton.EDU" appended to it.

• If you want others to be able to address their mail to user@foo, but that email should actually be delivered to another host named bar, then specify "bar".

  This will cause host foo to receive an MX record pointing to bar.

  If host bar is in the same DNS-DOMAIN as foo, then bar should be an unqualified or partially-qualified DNS domain name. It will be interpreted relative to foo's DNS-DOMAIN. E.g. if your host is johnboy in DNS-DOMAIN Princeton.EDU and you want the MX record to point to "grandma.walton.Princeton.EDU", then just enter "grandma.walton".

  If host bar is not in exactly the same DNS-DOMAIN as foo, then bar should be a fully-qualified DNS domain name, ending with a period; e.g. "bar.baz.ucb.edu.". The resulting MX record will then be "bar.baz.ucb.edu."; ending the MAIL-METHOD name with a dot prevents the DNS-DOMAIN from being appended when the MX record is constructed.

  Host bar must be the primary (a.k.a "canonical") name for a host, not an alias. If you enter an alias, mail may not be sent to your host reliably.

  Host bar must be prepared to accept email; i.e. it must run an SMTP server, and indicate so by having an MX record that points to itself (i.e. if host bar is at Princeton, its MAIL-METHOD is "bar"). Furthermore, bar must be prepared to accept email that is addressed to user@foo; that may require additional configuration on bar.

For either of the second or third approaches above, keep in mind that the target of the MX record (foo in the second approach, bar in the third approach) should be a machine that is essentially available round-the-clock. Machines may attempt to send email to the target of an MX record at any time, and if that target is unavailable, the sending machine will typically bounce the email back to the sender, often after trying for some period (e.g. 72 hours).

Note that the MX record is generated only for the entry's canonical name. If the entry has any aliases (regular host aliases, which are created with the ALIAS field), those generate CNAME records in DNS which point back to the host's canonical name (which in turn then has an MX record). If the entry has any interface aliases (those attached to specific interfaces, as opposed to host aliases), those interface aliases do not get any MX record. Since an interface alias results in a DNS A record (not a CNAME back to the host's canonical name), the interface alias does not inherit any MX record the host may have. If you need to specify an MX record for an interface alias, contact hostmaster@princeton.edu .

Support for multiple MAIL-METHOD values is limited. The Web interface cannot be used to add additional MAIL-METHOD values. If an entry has multiple MAIL-METHOD values, specifying a MAIL-METHOD value of NONE will remove all of these values; specifying any other value will replace the first value and leave the remainder alone. All of the MX records generated in DNS as a result of MAIL-METHOD values in Host Database entries will have the same MX preference value.

Dormnet Subscribers

Although the electronic forms used to subscribe to Dormnet (or change a Host Database entry associated with a Dormnet subscription) do not let you modify your MAIL-METHOD, you may request that your entry's MAIL-METHOD be changed, by sending email to hostmaster@princeton.edu . (This field is not available on the Dormnet forms because changing it is almost always the wrong thing to do for student machines.)

Before changing this field, you should be sure you understand the information above, and think about the ramifications. It is almost always a mistake for a student to modify his or her entry's MAIL-METHOD. (Few students install SMTP servers on their computers that allow them to accept email, and keep their computers online round-the-clock, year-round.)

When a Dormnet subscription is unsubscribed, any MAIL-METHOD it had is automatically removed. If you wish for it to remain, you will need to contact OIT afterwards to request it be re-inserted.