This is just one example how short-term DNS inconsistencies can lead to temporary SPF failures and bouncing of legitimate mail. As a result, the SPF check fails, and the message bounces. The resolver queries one of the authoritative name servers for and obtains the new MX record (which already points to mail.lf.net). What happens? It turns out that due to the caching nature of DNS, resolvers at some store the old SPF record in their caches, but not the MX records, When an SPF-aware MTA receives mail from, it tries to obtain the MX record for that domain from its local resolver.

However, when you do this, your customers will soon start to phone and complain to you that they cannot send mail anymore to some destinations. The SPF record is updated accordingly, so that still can be used as an outgoing mail relay: .IN TXT'v=spf1 a: -all' .IN MX10 mail.lf.net. Now suppose that the administrator discovers that he cannot handle the growing mail load anymore, and therefore migrates mail handling to the ISP's mail server, mail.lf.net. This means that SPF-aware receiving mail transfer agents accept mail message. is also the outgoing mail relay for clients sending mail from the domain. Suppose publishes the following DNS records: .IN TXT'v=spf1 mx -all' .IN MX10. Let's look at an example to see what this means. DNS is distributed database with an optimistic replication protocol, which trades temporary lack of consistency for increased availability.

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