Choosing the RHS of Secondary MX Records
(Based on a post to comp.mail.sendmail on 18 Jul 2003.)
An important step in setting up a secondary mail server is adding a
second MX (mail exchanger) record to the set of DNS records for the domain:
vmailpros.com. 1D IN MX 10 mail.vmailpros.com.
vmailpros.com. 1D IN MX 50 mail2.vmailpros.com.
In the above example, mail.vmailpros.com is the host name of the
primary mail server for the domain vmailpros.com; mail2.vmailpros.com
is the host name of the secondary mail server.
Most instructions for setting up MX records indicate that the name
of the mail server (on the RHS [right-hand-side] of the MX record) must be
the actual name of the host receiving mail. While this is a true
statement, in practice if there are multiple "A" records that point to
the primary mail server, any of them can appear on the RHS of the MX
record in most cases.
However, for secondary, tertiary (etc.) mail servers, it is
critical that the RHS of the MX record be the actual
host name of the secondary server. Otherwise, you will get the
infamous "mail loops back to myself" error.
What Is the Real Name of My Secondary Mail Server?
From the command line, try these:
- Type: hostname
- Type: echo '$j' | sendmail -bt
- If you don't like the above, try adding this to your mc file and
build a new sendmail.cf:
where myCanonicalName is the host name you want sendmail to
use as its real name.
Scenario: Why Mail Loops When You Use the Wrong Host Name
- The primary mail server for the address is down.
- The sending client or intermediate mail server looks up the
secondary server and sees that its name is dude.vmailpros.com, which
resolves to the same IP address as the server whose "real" name is
mail2.vmailpros.com. The mail is sent to mail2.vmailpros.com (the same
server as dude.vmailpros.com.)
- The server mail2.vmailpros.com looks in relay-domains or access.db
and sees that it relays mail for the target domain, so it accepts the
- mail2.vmailpros.com tries to decide what to do with the mail. It
first tries to contact the primary mail server, which is still down.
- mail2.vmailpros.com sees that dude.vmailpros.com is the secondary
server. mail2.vmailpros.com looks in $j and doesn't see
dude.vmailpros.com (but mail2.vmailpros.com), so it needs to use the
SMTP mailer to send the mail to dude.vmailpros.com.
- Using the smtp mailer, mail2.vmailpros.com opens a connection to
dude.vmailpros.com (which is itself!!) and sends the mail (to
- Go to step 3 (until maximum MX loops reached, then fail).