How long does it take an MX record to propagate around the internet?

Installed new exchange server 2007. Once I create the MX record, how long does it take to be recognized around the internet?
mikeymail305Asked:
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Dave HoweSoftware and Hardware EngineerCommented:
for a new domain? almost instantly :)

the way it works is, for each *new* record, the first time a server sees an email to that domain, it will request the MX record(s) from the dns server holding it/them.  It will then hold its copy for whatever default TTL is set to in your SOA record.

if it is *not* a new record, then the answer is usually about twice the default TTL. That is explained by the fact that most companies don't query the real name server directly, but query their isp's resolver (called a forwarder) for the information, which (if it doesn't have a copy) queries your server.

Now that's ok, apart from the case where customer A queries the name server and gets an answer. if *immediately* afterwards, you change the record, then both the forwarder and company A will continue to believe the old record for one TTL (usually one day)

however, should a second customer (customer B) query the nameserver before the TTL is up, it will be given the old answer - and *it* will now believe that for one TTL period. its a day old, stale record, but no matter - it will still hold onto it for one TTL.

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