Mx, nomx, nodns, defaultmx, randommx, nonrandommx, affinitylist, nameservers, defaultnameservers Channel Options

TCP/IP nameserver and MX record support
Most TCP/IP networks support the use of MX (mail forwarding) records for SMTP relay but a few do not. The MTA&#x27;s TCP/IP channel programs can be configured to not use MX records if they are not provided by the network to which the MTA system is connected. Some TCP/IP channel programs can be configured to not do DNS (nameserver) lookups at all. specifies that MX lookups should be done and MX record values of equal precedence should be processed in random order. specifies that MX lookups should be done and MX values of equal precedence should be processed in the same order in which they were received. The  option is currently equivalent to  ; it may change to be equivalent to   in a future release. The  option disables MX lookups. The  option specifies that   should be used (with randomization) if the network says that that MX records are  supported, and if the destination port is port 25 for SMTP; (with , destination ports other than port 25 do not get MX lookups by default).

LMTP channels do not normally make use of MX lookups. Additionally, LMTP channels need to connect to the correct store taking failover events into account. As of the 8.0 release, the   channel option provides this functionality. disables MX lookups completely and translates the logical host given into the corresponding affinity group. Connections are then attempted sequentially to all the hosts in the group.

The default is  on channels that support MX lookups in any form.

New in MS 8.0 is specialized handling for MX entries of the form: nomail         IN MX 0. Such entries are intended to be an indication that host "nomail" does not operate a mail server. So when MX lookups are enabled, attempts to send to such a host will fail immediately after the MX lookup instead of attempting any sort of A record lookup. (Additionally,   will treat such hosts as not being a valid source of mail.)

Whether the operating system&#x27;s TCP/IP local host tables are used in addition to the DNS for lookups is generally controlled by  or the equivalent. Generally, operating system distributions are configured so that local host tables will indeed be consulted.

When nameserver lookups are being performed, that is, unless the  file selects no use of nameservers, then prior to Messaging Server 7.0  the   channel option may be used to specify a list of nameservers to consult rather than consulting the TCP/IP stack&#x27;s own choice of nameservers. This affects the SMTP server and client and LMTP client, but not the LMTP server (which, if it needs to do any lookups, always relies on the TCP/IP stack&#x27;s own choice of nameservers). Futhermore, as of Messaging Server 7.0, the  channel option only affects MX record lookups, with all other lookups using the TCP/IP stack&#x27;s choice of nameservers regardless of any   channel option setting. requires a space separated list of IP addresses for the nameservers, e.g., 1.2.3.1 1.2.3.2 is the default, and means to use the TCP/IP stack&#x27;s own choice of nameservers.

Note that while a  setting is primarily meaningful to the SMTP client -- hence TCP/IP destination channels -- it also, prior to Messaging Server 7.0, potentially had meaning to the SMTP server -- hence TCP/IP source channels -- as for instance in cases where the SMTP server channel had been configured to perform DNS reverse lookups on incoming connections, or to perform forms of DNS verification.

See also:
 * TCPIP channels
 * dnsforcetemporary Option
 * mailfromdnsverify Option
 * TCPIP connections and DNS lookups channel options
 * Channel options