sendmail, 4th Edition

MX Records Are Nonrecursive

Consider the following MX setup, which causes all mail for hostA to be sent to hostB and all mail for hostB to be sent to hostB, or to hostC if hostB is down:^[156]

hostA   IN     MX  10 hostB
hostB   IN     MX  10 hostB
        IN     MX  20 hostC

One might expect sendmail to be smart and deliver mail for hostA to hostC if hostB is down. But sendmail won’t do that. The RFC standards do not allow it to recursively look up additional MX records. If sendmail did, it could get hopelessly entangled in MX loops. Consider the following:

hostA   IN     MX  10 hostB
hostB   IN     MX  10 hostB
        IN     MX  20 hostC
hostC   IN     MX  10 hostA     ← potential loop

If your intention is to have hostA MX to two other hosts, you must state that explicitly:

hostA   IN     MX  10 hostB
        IN     MX  20 hostC
hostB   IN     MX  10 hostB
        IN     MX  20 hostC

Another reason sendmail refuses to follow MX records beyond the target host is that costs in such a situation are undefined. Consider the previous example with the potential loop. What is the cost of hostA when MX’d by hostB to hostC? Should it be the minimum of 10, the maximum of 20, the mean of 15, or the sum of 30?

^{[156] †} We are fudging for the sake of simplicity. Here, we assume that all the hosts also have A records.