Soft bounce
Temporary delivery failure — mailbox full, transient network issue, MX unreachable.
A soft bounce is an SMTP response in the 4.x.x range (e.g. 421, 450, 451, 452) indicating that delivery failed right now but a retry might succeed.
Typical causes: - 4.2.2 — mailbox full - 4.4.1 — receiving server temporarily unavailable - 4.7.0 — temporary rate-limit on the recipient side - 4.5.3 — too many recipients
Sendersy (via Postal MTA) automatically retries up to 5 times with exponential backoff. If all 5 attempts fail, the soft bounce is converted to a hard bounce and the address is added to the suppression list.
A high soft-bounce rate (>2%) signals that your list contains many inactive addresses. Clean the list regularly via preference center or periodic re-engagement campaigns.