- 4xx bounce code, defined
- A 4xx SMTP reply is a temporary failure telling the sender to try again later, and it says nothing about whether the mailbox exists.
The usual causes are mundane: greylisting on a first contact, a rate limit after too many connections, a mailbox over quota, or a server too busy to talk. Each returns a similar code with different text.
A 451 typically means a local processing error or a greylist. A 421 means the service is not available and the connection is about to close. A 452 means insufficient storage, which is often a full mailbox.
Sending platforms retry on a schedule over 24 to 72 hours, then convert what never clears into a permanent failure. That conversion is where soft bounces quietly become hard ones in your reporting.
For a verifier the honest handling is to retry at least once after a delay and to report the address as unresolved if the server stays evasive. Guessing is cheaper and produces a better-looking accuracy figure.
How ZapBounce reports it
First 4xx triggers a retry rather than a verdict. If the second pass is still temporary, the result is unknown with the code in smtp_code and the reason temporary_failure, and it is not billed, because nothing about the mailbox was established.
Three providers saying slow down, in their own words
Temporary refusals from the large providers are rarely about a mailbox. Gmail's rate limit reads 421 4.7.28 and mentions an unusual rate of unsolicited mail. Microsoft 365 sends 451 4.7.500 Server busy, please try again later. Yahoo defers with 421 4.7.0 and a bracketed tag such as TSS04, adding that messages are temporarily deferred due to unexpected volume or user complaints.
All three have a 7 in the middle of the enhanced code, which marks a security or policy decision. Set that beside 452 4.2.2, where the middle 2 points at the mailbox and the usual cause is a full inbox. A retry helps with both, but what you ought to do differs.
Say a campaign gets 4.7.x deferrals on 30% of its Yahoo traffic. Retrying harder makes things worse, because volume was the complaint. Cut your send rate to that provider and look at what changed recently in your list or your complaint numbers.
How long a message waits before it becomes a bounce
A deferred message sits in the sending server's queue and gets retried at growing intervals: a few minutes at first, then every hour or longer. Mail server software traditionally keeps trying for around five days before giving up, in line with what the SMTP standard recommends. Commercial sending platforms cut that to a day or three, since marketing mail goes stale.
During that window your reports can mislead you. The message hasn't been delivered and hasn't bounced, and many dashboards tuck that state into a small deferred column that nobody reads.
Verification meets the same codes under tighter constraints. A probe can't wait five days. It can retry after minutes, and if the server still won't commit, the accurate label is unknown with the code attached. When a 4.7.x reply shows up during a probe, it tells you something about the prober's standing with that server and nothing about the address.
4xx bounce code: common questions
How long should a 4xx be retried?
Sending platforms retry across 24 to 72 hours. For verification, one retry after a short pause clears most greylisting.
Is a 4xx a bounce?
It is a soft bounce. The message was not accepted this time, and the same message may well be accepted on the next attempt.
What does 421 mean specifically?
The service is unavailable and the connection is closing, usually from load or rate limiting. It is about the server, not the address.