4.4.5 deferral: mail system congestion

What it means

RFC 3463 calls X.4.5 "Mail system congestion". It is a network-level busy signal: a server or the path to it cannot take more mail at the moment and asks you to come back later. The subject 4 places it in routing and network status rather than on one mailbox or on your message.

Sendmail-style servers typically answer 452 4.4.5 when the queue disk runs short, as in the example, and gateways may send 421 4.4.5 when connection or concurrency limits are reached. It is close to 4.3.1, which says the system is out of storage, and to 4.7.x throttling replies, which target a particular sender’s rate rather than overall load.

The 4 class is almost always used, since congestion clears; your queue retries with growing intervals. A 5.4.5 is unusual, so if you see one, check whether the sending software converted the last deferral into a permanent failure at queue expiry.

Likely causes

• The receiving server hit its maximum number of concurrent connections or processes.
• Its queue disk dropped below the free space reserve, so new messages are deferred.
• A spam wave or a large legitimate campaign is flooding the receiver.
• Your server opens too many parallel connections to one destination and trips its limits.

Checks that apply

• SMTP Diagnostics Shows whether the recipient server greets normally now or still answers with a busy or deferral message.
• MailFlow Monitor Monitors delivery of test mail over time, so you can see when the congestion clears.

FAQ

How long should I wait after 4.4.5?

Nothing to do: your queue retries on its own. Most congestion clears within a few hours.

Is 4.4.5 a block against me?

Not usually. It reflects load, although sending fewer parallel connections to that host helps.