What Is a Delivery Deferral?
TL;DR
A delivery deferral is a receiving server returning a temporary 4yz SMTP reply instead of accepting a message, which tells the sending server to keep it queued and try again later. Greylisting, rate limiting, a full mailbox and a server under load all produce one, and most deferred mail is delivered on a later attempt.
What causes a deferral?
A condition the receiver expects to pass. RFC 5321 distinguishes a permanent failure, which will happen again for the same address, from a temporary one, where the address might be accepted if the client tries again later, and a deferral is the second of those. Greylisting, rate limits, load and an over-quota mailbox are the everyday causes.
Some deferrals are reputation decisions in temporary clothing. A receiver that is not ready to commit to a new sender can hold mail with a 4yz reply rather than refuse it, which looks identical in the logs and behaves differently: it does not clear on its own.
What should you do about deferrals?
Nothing per message, because the queue is already doing the right thing, and resending by hand only creates duplicates. What deserves attention is the shape of the deferrals rather than their existence.
Read them by receiver. Deferrals scattered across many domains usually mean your own provider is throttling output; deferrals concentrated at one large provider mean that provider is throttling you, and the response is to lower volume there rather than to retry harder.
Frequently asked questions
They describe the same SMTP reply from two positions. The receiver defers; the sending platform usually records it as a soft bounce. The distinction that matters is between a temporary 4yz reply and a permanent 5yz one, not between the two words.