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Managing email bounces
The different bounce types in MailMachine - soft, hard, blocked, full mailbox and more - and how to handle each.
A bounce is a failed delivery attempt. A soft bounce is a temporary failure that delays delivery but keeps the contact on your list; a hard bounce means the address is unreachable and is removed. Knowing the different bounce types helps you read your reports and keep your list healthy. For the metrics and benchmarks, see Bounces, complaints and unsubscribes.
The bounce types you'll see
Soft bounce - the message reached the recipient's server but came back undelivered, often a temporarily full inbox. Repeated soft bounces to the same address eventually become a hard bounce.
Transient bounce - the sending server is still retrying; it keeps trying for up to two days. Usually nothing to worry about. You may see "message still undelivered after 4 hours".
Address change - the recipient has moved to a new address and the server is letting you know.
Auto-reply - an out-of-office or similar automated response. Not a real bounce; safe to ignore, though useful to know if your message was time-sensitive.
Challenge/Response - filtering software that only accepts mail from known senders has asked the sender to complete a challenge before the message is delivered.
DNS failure - a temporary DNS problem stopped delivery.
Hard bounce - the address is invalid: the domain doesn't exist, or the mailbox is unknown. Removed from your list.
Mail blocked - the recipient's server is refusing your mail (you might see "550 Message REFUSED by peer" or "552 Blocked by filters").
Full mailbox - the recipient's inbox is full, so the server can't deliver.
Troubleshooting
Hard bounce rate suddenly jumped - usually a freshly imported list that wasn't validated, or typos in manually added addresses. Validate your list before the next send.
The same addresses keep soft-bouncing - watch whether they turn into hard bounces, check if a whole domain is bouncing (which can mean blocking), and consider removing addresses that soft-bounce across several campaigns.
Lots of "full mailbox" bounces - these accounts may be abandoned. Run a re-engagement campaign, and drop addresses that stay full for a month or more.
Challenge/Response is blocking delivery - only worth completing the challenge for genuinely high-value contacts; otherwise these are high-effort to maintain.
If bounces spike for no obvious reason, open a support ticket.