- Soft bounce, defined
- A soft bounce is a temporary delivery failure: the server could not accept the message right now but might accept the same message later.
Soft bounces come back as 4xx codes. A full mailbox, a server under load, a message larger than the recipient allows, and a greylisting delay all look the same at this level.
Sending platforms retry soft bounces on a schedule, usually over 24 to 72 hours, before giving up and converting the address to a permanent failure.
A soft bounce that repeats for weeks is worth treating as abandoned even though the server never said so. Nobody is emptying that mailbox.
How ZapBounce reports it
A 4xx during verification is not a verdict, so we retry rather than guess. If the server is still evasive on the second pass we return unknown and do not bill it, because we never established anything.
A full mailbox across three sends
Picture a monthly newsletter going to dana@ at a small architecture firm. In March the server answers 452 4.2.2 and says the account is over quota. Your platform retries for two days, gives up, and logs a soft bounce. April brings the same code, and so does May.
Nothing in those three replies says the address is wrong. The 4 at the front means temporary, and 4.2.2 is the enhanced code for a mailbox that's out of storage. Dana may be on leave with a full inbox, or she may have left the firm in February while IT kept the account alive for forwarding.
You can't tell which from the code alone, and neither can a verifier. A probe sent during that period gets the same temporary answer, so the right label is unknown. Your own send history is the better evidence here: three straight months of 4.2.2 is a pattern no single check can see.
Writing a rule for the ones that never clear
Most platforms let you set how many consecutive soft bounces trigger suppression, and the default is often generous. Count sends, not days. Three failures on a daily list cover half a week, while three on a monthly list cover a quarter, and only the second tells you much.
Split the rule by reason if your platform exposes the enhanced code. A 4.2.2 that repeats points at an unattended mailbox, which is about the recipient. By contrast, a 4.7.x reply is a policy or rate decision about you, and suppressing the recipient for it throws away a good address while leaving the real problem untouched.
Watch the ratio as well. When soft bounces at one provider jump from 0.5% to 6% in a single campaign, you're being throttled there, and the fix is to slow down.
Soft bounce: common questions
How many times should a soft bounce be retried?
Sending platforms typically retry over 24 to 72 hours. For verification, one retry after a short delay resolves most greylisting without hammering the server.
When does a soft bounce become a hard bounce?
When the retry window expires. Most platforms convert the address to a permanent failure after roughly 72 hours of temporary rejections.
Does a full mailbox mean the person left?
Not necessarily, but a mailbox that stays full for weeks is usually unattended. Treat it as disengaged rather than invalid.