421 4.4.5 "Server Busy": Why the Receiving Server Closed the Connection

421 4.4.5 means the receiving server is too busy to accept your message right now and has closed the connection. The text is usually "Server busy, try again later." It is a soft bounce: your server retries on its own, and the code implies nothing about the address or your reputation.

The 421 4.4.5 code page has the summary.

Reading the code

Part	Value	Meaning
Reply code	421	Service not available, closing the channel
Class	4	Persistent transient failure
Subject	4	Network and routing status
Detail	5	Congestion

RFC 3463 defines X.4.5 as "Mail system congestion". The 421 reply code means the server is closing the transmission channel, so the sender has to reconnect later.

Possible causes

• The receiving server is handling more connections than it can take.
• Your server opened many connections to the same provider at once.

What to do

1. Let the queue retry. A busy signal clears by itself most of the time.
2. Lower concurrency if you open many connections at once to one provider. What is email throttling explains why senders slow down on purpose.
3. Do not suppress the address. A deferral is not a verdict on the mailbox. See soft bounce handling.
4. Read the whole message. Some providers send a similar deferral for reputation reasons, for example Gmail 421 4.7.28 or Microsoft 451 4.7.650. Those have different codes and different fixes.

How to tell busy from blocked

Signal	Likely meaning
4.4.5 with plain "busy" text	Capacity on their side or too many connections from you
4.7.x with reputation or rate wording	The provider is slowing you because of how your mail looks
Same message repeats for days	Look at the specific text, not just the code

The enhanced code subject digit helps here: 4 is about the network and 7 is about security or policy. The enhanced status codes page lists them.

Where verification helps

Only partly. Temporary deferrals are not about the address, but providers defer senders whose mail looks risky, and a high rate of hard bounces is one of the signals they use. Sending to real mailboxes keeps that signal quiet.

Keep the address side clean

This code does not come from your list, but every bounce still counts against your sending reputation, so the addresses you do send to should be real. Upload your list on the verify page: the first 100 email checks are free with a Google sign-in, no card. Each address comes back valid, risky or invalid with a reason such as "Mailbox not found" or "Accepts all addresses", and nothing is sent to anyone on the list. For larger or recurring lists, see bulk email verification, list cleaning and pricing.