Recently, the email sending industry saw some fairly significant issues with delivering email to Yahoo. Senders trying to deliver bulk emails to their Yahoo subscribers found these to be continually “deferred”, or temporarily bounced back. It was later discovered that Yahoo had upgraded their filtering systems, which caused this continuous deferring. Since then, they have fine-tuned their systems, and are reporting that the issue has been resolved. Yahoo also noted that any sender still experiencing their mail being deferred is an issue that the sender will need to resolve. I would like to discuss both the mechanics of mailing systems, with respect to deferring, as well as why some ISPs (like Yahoo) choose to defer messages.

MTAs Explained

To the average individual, sending email is all about composing a message and clicking the “send” button. The message then is somehow delivered to the recipient. However, there is a whole lot of technology that happens in those few seconds when the message is transmitted. Here’s a simplified process flow:

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