Definition
Soft bounce: A temporary delivery failure signaled by a 4xx reply, for example a full mailbox, a busy server, rate limiting or greylisting. The address may still be valid, so the message is retried; repeated soft bounces over several sends are usually treated as hard.
Common causes of soft bounces
A full mailbox is the classic one, usually 452 with the enhanced code 4.2.2. Rate limiting is next: large providers defer mail with 421 replies when a sender pushes too much too fast or has a weak reputation.
Greylisting defers the first attempt from an unfamiliar sender on purpose, and a busy or briefly unreachable server produces the same kind of temporary reply.
How retries work after a soft bounce
RFC 5321 has the sending server keep the message in its queue and try again. It says the retry interval should generally be at least 30 minutes and that the give-up time generally needs to be at least 4 to 5 days.
Email platforms set their own schedules inside that frame. Most retry for a period of hours to days and then report the message as bounced if every attempt fails.
When a soft bounce should become a hard bounce
One soft bounce means nothing. The same address soft-bouncing on several consecutive campaigns usually means an abandoned mailbox that has filled up and will never be emptied.
Many platforms convert an address to a hard bounce after a set number of consecutive soft bounces. If yours does not, set up a rule that does.
C: RCPT TO:<sam@example.com>
S: 452 4.2.2 <sam@example.com>: Mailbox full, try again laterHow RocketVerifier handles it
Temporary answers never turn into a guess. A greylisted check is retried automatically, up to 3 times about 10 minutes apart, and a check that still ends without an explicit answer comes back as unknown and is never charged. A full mailbox is reported as risky with sub_status mailbox_full, because the address exists but mail to it will keep bouncing until the owner clears space.