The concept of multiple MX records is simple enough: if your primary mailserver is down, anyone sending mail can look to the other record or records and send mail there. Sounds like a good idea, right? Actually, for most of us it is not. I've recently had two customers have problems that demonstrate why most small businesses should have one and only one MX record.
The first involved another party who had recently switched their mail to one of Microsoft's lovely offerings. Immediately after that, mail to my customer's domain from that party ALWAYS went to the secondary MX. That's a fairly minor annoyance, but it did cause delays and this happens to be time critical messages. It also caused a spam problem as the secondary saw the Microsoft address as a source of spam and although my customer could whitelist that mail at his Kerio server, he couldn't at the secondary. Sometimes that's the other way around: the secondary may not see something as spam that your server would.
It gets worse, though. Another customer, overcome by power, had FIVE MX records. Each had its own logins and passwords and each had its own mail quirks. Some use POP3, some use ETRN and some deliver via port 25 when they see the primary has become available. That caused untold confusion when he switched ISP's.
Understand also that secondaries WILL be used. Spammers may deliberately target you through your other records and broken DNS may send back the wrong answer to a MX query. Also, any momentary glitch in connectivity may cause a sender to use a secondary.
In the days of UUCP and flaky internet, multiple MX records made sense. Today, in most areas, our internet connections are stable and reliable. If we do lose connection, other machines will keep trying - there's really no need to have a secondary unless you will be down for days. If and when that happens, add or just change your MX then.
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