It is now time to consider the mailhost and mailRoutingAddress attributes. The actual attributes considered can be overridden with the LDAP_MAILHOST and LDAP_ROUTING_ADDRESS MTA options, respectively. These attributes work together to determine whether or not the address should be acted on at this time or forwarded to another system.
The first step is to decide whether or not mailhost is meaningful for this entry. A preliminary check of the delivery options active for the entry is done to see whether or not the entry is mailhost-specific. If it is not, mailhost checking is omitted. See the description of 18.104.22.168 Delivery Options Processing, and the # flag in particular, to understand how this check is done.
In the case of a user entry, the mailhost attribute must identify the local system in order to be acted on. The mailhost attribute is compared with the value of the local.hostname configutil parameter and against the list of values specified by the local.imta.hostnamealiases configutil parameter. The mailhost attribute is deemed to identify the local host if any of these match.
A successful match means that the alias can be acted on locally and alias processing continues. An unsuccessful match means that the message needs to be forward to the mailhost to be acted on. A new address of the form
is constructed and becomes the result of the alias expansion operation.
The handling of a missing mailhost attribute is different depending on whether the entry is a user or a group. In the case of a user, a mailhost is essential, so if no mailhost attribute is present a new address of the form
is constructed using the smart host for the domain determined by the LDAP_DOMAIN_ATTR_SMARTHOST MTA option. An error is reported if no smart host exists for the domain.
Groups, on the other hand, do not require a mailhost, so a missing mailhost is interpreted as meaning that the group can be expanded anywhere. So alias processing continues.
The mailRoutingAddress attribute adds one final wrinkle. Its presence causes processing to terminate with the mailRoutingAddress as the result. In version 5.2, the mailHost check was done first and must pass before the routing address can take effect. To get the same behavior in the current version of Messaging Server, the format of the mailRoutingAddress attribute could be as follows: mailRoutingAddess: @mailhost:user@domain