There are a couple of special issues left out of the preceding section because they complicated the discussion. They are treated here in no particular order.
Wildcard names, those containing the character '*' in them, may be used for mail routing. There are likely to be servers on the network which simply state that any mail to a domain is to be routed through a relay. For example, at the time that this RFC is being written, all mail to hosts in the domain IL is routed through RELAY.CS.NET. This is done by creating a wildcard RR, which states that *.IL has an MX of RELAY.CS.NET. This should be transparent to the mailer since the domain servers will hide this wildcard match. (If it matches *.IL with HUJI.IL for example, a domain server will return an RR containing HUJI.IL, not *.IL). If by some accident a mailer receives an RR with a wildcard domain name in its name or data section it should discard the RR.
Note that the algorithm to delete irrelevant RRs breaks if LOCAL has a alias and the alias is listed in the MX records for REMOTE. (E.g. REMOTE has an MX of ALIAS, where ALIAS has a CNAME of LOCAL). This can be avoided if aliases are never used in the data section of MX RRs.
Implementors should understand that the query and interpretation of the query is only performed for REMOTE. It is not repeated for the MX RRs listed for REMOTE. You cannot try to support more extravagant mail routing by building a chain of MXs. (E.g. UNIX.BBN.COM is an MX for RELAY.CS.NET and RELAY.CS.NET is an MX for all the hosts in .IL, but this does not mean that UNIX.BBN.COM accepts any responsibility for mail for .IL).
Finally, it should be noted that this is a standard for routing on the Internet. Mailers serving hosts which lie on multiple networks will presumably have to make some decisions about which network to route through. This decision making is outside the scope of this memo, although mailers may well use the domain system to help them decide. However, once a mailer decides to deliver a message via the Internet it must apply these rules to route the message.