What are common soft bounce reasons?

A soft bounce is the receiving server saying "not now, try again later." The address probably exists. Something on the other end is refusing this specific delivery attempt. Your mail server sees a 4xx SMTP response and retries on a schedule until it either gets through or gives up and converts the bounce to permanent.

Here are the reasons you actually run into.

Mailbox full. The recipient hasn't cleaned their inbox and the provider is out of room. SMTP code 452 ("insufficient system storage") is the classic one. Most providers retry for a couple of days, then convert it to a hard bounce.

Greylisting. The receiving server rejects the first attempt with a 451 on purpose, betting that spam software won't retry. Real mail servers do retry, so the second or third attempt goes through. You see this on smaller providers and corporate domains running Postfix or sendmail.

Rate limiting and throttling. You sent too fast or too much from one IP. Gmail, Yahoo, and Microsoft all throttle when your reputation isn't strong enough to support the volume you're pushing. The response is usually a 421 with a "try again later" message, or a Yahoo TS deferral. The fix is slower sends, better warmup, or improving the reputation that triggered the throttle. See how hygiene issues affect sender reputation.

Temporary server errors. The receiving mail system is having a bad day. Maintenance, overload, a misconfigured queue. You see 421 ("service not available") or 451. Nothing you can do but let the retries run.

Transient DNS or network failures. Your sender couldn't resolve the MX record, or a TCP connection dropped mid-conversation. Nothing wrong with the address. The retry sorts it out.

Connection limits per IP. Big mailbox providers cap how many concurrent SMTP connections one IP can hold open. Hit the cap and the next connection gets refused with a 421.

Policy-based deferrals. The provider doesn't want the message right now for a reason that isn't quite a hard rejection. Spam filter uncertain, content flagged for review, recipient on a temporary block. Microsoft does this aggressively with "421 4.3.2 Service unavailable" responses tied to reputation.

What soft bounces actually look like

All soft bounces come back as 4xx codes per RFC 5321 section 4.2.1. The ones you'll see most often:

• 421 Service not available, closing transmission channel
• 450 Mailbox unavailable, possibly busy
• 451 Local error in processing (greylisting often uses this)
• 452 Insufficient system storage (mailbox full)

There is no 422 in SMTP. 422 is an HTTP status code (Unprocessable Entity). If you see "422" in a bounce log it's either a sender misreporting or someone confusing protocols. Don't chase it.

Why soft bounces matter for hygiene

A one-off soft bounce is meaningless. A pattern of soft bounces on the same address week after week is a hygiene signal. Mailbox-full bounces that never resolve usually mean the recipient abandoned the inbox. Rate-limit deferrals across a whole campaign mean your reputation is slipping and you should look at how often you're cleaning your list.

Most ESPs convert a soft bounce to a hard bounce after a set number of consecutive failures. Mailchimp does 7, Klaviyo uses 7 days of repeated failures, others vary. Once it converts, the address is suppressed. That's fine. What's not fine is treating soft bounces as background noise and never noticing the pattern. Track your soft-bounce rate the same way you track list quality overall, and watch for the silent ones described in hidden engagement decay.