A soft bounce is a temporary email delivery failure returned with a 4xx SMTP code: the mailbox is full, the receiving server is busy or greylisting, the message is too large, or a policy check deferred it. The address itself may be fine, so sending platforms retry for a period before giving up. An address that soft-bounces on three consecutive campaigns should be treated as a hard bounce, since a mailbox that is always full is usually abandoned.
Soft bounces are counted per message attempt and reported separately from hard bounces by most platforms. They contribute less to reputation damage than hard bounces, but a high soft bounce rate on a single provider usually means that provider is throttling you, which is a reputation signal in itself.
A verifier sees the mailbox state at the moment it probes. A full mailbox may show as valid (the address exists) or unknown (the server deferred the probe). What verification removes is the hard-bounce population; soft bounces are managed by the sending platform's retry and suppression rules.
A campaign of 10,000 verified addresses returns 40 hard bounces and 310 soft bounces, 260 of them deferrals from one ISP that greylists new senders. Retries deliver 240 of the 310 within a day. The 70 that never deliver are suppressed after the third campaign, and the sender slows its ramp on that ISP.
Verify a list or a single address. 100 free checks a month, unknowns refunded.
Verify Leads - FreeAyoub built BounceZero's 5-stage validation pipeline, its dedicated BGP-announced IP infrastructure, and the Patroni HA PostgreSQL cluster behind every verification. Previously built high-volume email delivery infrastructure. Trained at 1337 Benguerir (École 42 network, 2019). Open-source: bgp_analyzer.