Soft bounce

A soft bounce is a temporary delivery failure — the receiving mail server accepted the connection but rejected the message for a transient reason such as a full mailbox, a too-large attachment, or a brief throttle.

What it is

SMTP defines two categories of negative responses. The 5xx class is permanent: the message will never be accepted. The 4xx class is temporary: the server is saying, in effect, "try again later." A soft bounce is any 4xx response. Common examples include 421 Service not available, 450 Mailbox temporarily unavailable, 452 Insufficient storage, and 451 Greylisted, please retry.

From the sender's perspective the message has not been delivered, but it also has not been permanently rejected. The right response is usually to queue it and retry — exactly what every well-behaved sending MTA does automatically.

Common causes

Soft bounces fall into a handful of buckets. Mailbox full is the classic — the recipient has not logged in for months and the storage quota is exhausted. Greylisting is a deliberate spam-filtering tactic where receivers reject every first attempt and only accept on retry, on the assumption that real MTAs retry and spam cannons do not. Rate-limiting kicks in when you've sent too quickly into a single domain. Server-side outages, DNS hiccups, and the recipient's domain being temporarily unreachable round out the list.

A subtler cause is reputation-based throttling. Gmail and Microsoft will respond with 4xx codes when they want to slow a sender down without rejecting them outright — for instance, when you ramp volume faster than the mailbox provider trusts.

Retry strategy

Standard practice is exponential backoff. Retry after 15 minutes, then an hour, then four hours, doubling until you hit a ceiling of about every six hours, for up to 48 to 72 hours total. After that, give up and treat the address as undeliverable for that send. Most ESPs and cold-email tools handle this automatically; you only need to intervene if you see a pattern.

When to remove the address

A single soft bounce is noise. Three to five consecutive soft bounces across a week is signal — the mailbox is probably abandoned, full permanently, or the domain is dead. Industry convention is to escalate any address with five consecutive soft bounces into the same suppression list you use for hard bounces. Sending to addresses that bounce every time is a fast path to a worse sender reputation, because mailbox providers read sustained bounce rates as a sign of a scraped or stale list.

Why it matters

Bounce rate is one of the inputs every mailbox provider uses to score senders. Keep your overall bounce rate — hard plus chronic soft — under 2 percent and you are in safe territory. Push above 5 percent for a campaign and Gmail and Outlook will start treating you as a list-quality risk. The single best thing you can do as a sender is suppress aggressively.

Related

• Hard bounce — the permanent counterpart
• Sender reputation — what sustained bounces erode
• Email deliverability — the umbrella metric
• Cold email deliverability checklist
• How NeverSpam keeps your list clean while warming