We’ve had some reports of email being delayed or not getting delivered at all today. During our first tests, we were not able to reproduce the issue. However, we have now been able to and the issue is resolved.
The problem had to do with the Sender Policy Framework (SPF), which we support. You can find out more about SPF at http://www.openspf.org/. In a nutshell, SPF helps reduce spam by making it harder for spammers to hijack email domains.
The best practices for setting up SPF is to trust a list produced by The Trusted Forwarder Global Whitelist (www.trusted-forwarder.org) which provides a global whitelist for SPF users. It provides a way of allowing legitimate email that is sent through known, trusted email forwarders from being blocked by SPF checks because the forwarders do not use some sort of envelope-from rewriting system.
Unfortunately, the trusted-forwarder.org domain is failing to resolve today. We do not know what the issue is, but their DNS servers are reporting SERVFAIL. This was causing our SPF system to report “Bad SPF Record” and to reject email with a temporary error. A temporary error means that the email delivery attempts will continue for up to 5 days in the hopes that the problem will be resolved.
We have removed the trusted-forwarder.org lookups from our system, and therefore have eliminated the temporary failures. Email that has been delayed is beginning to be delivered now.
Note: This issue ONLY affected domains without a valid SPF record. That is why we were not able to replicate it at first, as all of the domains we tested with had SPF properly setup.