Unix Tip: Mail loops back to me

Many systems administrators have run into errors in their syslog files that complain that mail is looping back, suggesting a possible MX problem. The common cause of this problem is that a server is receiving email for a domain that it doesn't recognize as its own. Then, when the server looks up the MX address for the intended target (in order to send the mail on its way), it notices that the MX record is one that identifies the mail exchanger as the system itself.

Here's an example of this type of message from the syslog file:

553 5.3.5 mail1.elsewhere.com. config error: mail loops back to me (MX problem?)

The cure for this type of problem is to configure the server to understand that it should accept mail for the particular domain. This can be done for sendmail by adding the domain to the /etc/mail/local-host-names file and restarting the sendmail service.

Recently, however, I ran into a situation in which this familiar scenario did not describe what was happening. Instead of running into "loops back to me" errors for domains for which the server should legitimately have been receiving mail, the server in question was collecting the errors for numerous unfamiliar domains. For most modern Unix servers, this issue rarely occurs because all recent versions of sendmail do not relay mail by default (i.e., they do not accept mail from outside the domain that is also destined for outside the domain). In this case, however, the server was configured to relay mail for authenticated users, so it was required to attempt delivery.



To understand why "loops back to me" errors occur, it is useful to think about the way that mail servers go about sending mail. Most email is addressed to user@domain. For example, sstocker@itworld.com. The server then needs to figure out what mail server is responsible for accepting mail for the particular domain. To do this, it requests the MX (mail exchanger) records for the domain in question. You can look up MX records yourself with nslookup or a similar tool as shown below.

> nslookup
Default Server:  ns1.local.com
Address:  10.1.1.11

> set querytype=mx
> elsewhere.com
Server:  ns1.local.com
Address:  10.1.1.11

elsewhere.com  MX preference = 20, mail exchanger = mail2.elsewhere.com
elsewhere.com  MX preference = 10, mail exchanger = mail1.elsewhere.com
mail1.elsewhere.com  internet address = 123.4.5.6
mail2.elsewhere.com  internet address = 123.4.5.7
> exit

In this example, the system "mail1.elsewhere.com" is the primary mail exchanger identified for the domain elsewhere.com. The system "mail2.elsewhere.com" is a lower priority mail exchanger (with a preference of 20) and will receive mail when the higher priority server is unavailable.

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