Deferral

What it means

A deferral is the sending side's record of a 4xx reply or a failed connection: the message is not delivered yet, and has not failed either. The MTA keeps it and retries on its schedule until it is delivered or the queue lifetime runs out. Common causes are greylisting, throttling, temporary DNS failures and servers that are busy or down.

Deferrals often come before blocks. Yahoo documents its TS deferrals for complaints, content, poor IP reputation and unusual traffic, and Google advises that when messages start being deferred, senders reduce volume until the error rate falls and then increase it slowly. The text of a deferral usually names its reason, so a rising share of deferred mail at one provider is worth reading closely.