cf/README for sendmail 8.12.3

Eric Allman of the Sendmail Consortium

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Normally, the $j macro is automatically defined to be your fully qualified domain name (FQDN). Sendmail does this by getting your host name using gethostname and then calling gethostbyname on the result. For example, in some environments gethostname returns only the root of the host name (such as "foo"); gethostbyname is supposed to return the FQDN (""). In some (fairly rare) cases, gethostbyname may fail to return the FQDN. In this case you must define confDOMAIN_NAME to be your fully qualified domain name. This is usually done using:
define(`confDOMAIN_NAME', `$w.$m')dnl