Guide: sending
Hard and soft bounces, and what to do about them
A bounce is a receiving server's answer that a message was not delivered. Reading the answer correctly decides whether you retry, stop mailing the address, or fix something on your side.
Last reviewed November 4, 2026
How a bounce is reported
During the SMTP conversation the receiving server answers with a three-digit reply code (RFC 5321). A code starting with 2 means accepted, 4 means a temporary failure the sender may retry, and 5 means a permanent failure.
Most servers add an enhanced status code (RFC 3463) such as 5.1.1: the first digit repeats the class, the second names the subject (1 for the address, 2 for the mailbox, 7 for security or policy) and the third the detail. A server can also accept a message and send a delivery status notification (a bounce message) later; that is an asynchronous bounce.
550 5.1.1 The email account that you tried to reach does not exist
452 4.2.2 The recipient's mailbox is full, try again later
421 4.7.0 Too many connections, try again laterHard bounces
A hard bounce is a permanent failure about the recipient: the mailbox does not exist (5.1.1), the domain does not exist or has no mail server, or the account was closed. Sending again will not succeed.
Stop mailing the address at once. Repeated sends to addresses that do not exist are one of the clearest signs of a poorly kept list, and mailbox providers take them into account when they judge your mail.
Soft bounces
A soft bounce is a temporary failure: a full mailbox (4.2.2), a server that is busy or unreachable, or a receiver that slows a sender down. The sending server retries over a period of hours or days before it gives up.
One soft bounce is no reason to remove an address. An address that keeps failing for weeks, for a reason that does not change, behaves like a hard bounce and should be treated as one.
Blocked messages
Some permanent refusals are not about the recipient at all. Codes in the 5.7.x range, and replies that mention spam, reputation, authentication or policy, say that the receiver does not accept mail from this sender, server or content.
Removing the recipient does not help here. Check SPF, DKIM and DMARC, your complaint rate, the content and the sending IP's reputation; a sudden rise in blocks is a warning about your sending, not about your list.
What to do with each
- Hard bounce: suppress the address permanently and never import it again.
- Soft bounce: let the retries run; suppress only after repeated failures over a longer period.
- Blocked: keep the address, investigate your authentication, content and reputation, and slow down if a provider is throttling you.
- Watch the bounce rate per campaign and per provider. A spike after an import usually means old or bought addresses.
How Mail.Land handles bounces
Mail.Land classifies bounces as hard, soft or blocked, and as unknown when a reply cannot be classified. A hard bounce adds the address to the global suppression list at once, so no further mail goes to it. A soft bounce normally suppresses the address for marketing mail for 30 days, while transactional mail to it is still sent; an address's first expiry or message-too-large bounce suppresses nothing, and temporary failures are retried before a bounce is reported. A blocked bounce suppresses nothing, because it says something about the sending, not the recipient.
Each bounce is visible in the email logs with its reason, and the message.bounced webhook carries the bounce type.
Sources
- RFC 5321, Simple Mail Transfer Protocol (checked November 4, 2026)
- RFC 3463, Enhanced Mail System Status Codes (checked November 4, 2026)
Put it into practice
Add your domain to Mail.Land and the dashboard shows the exact SPF, DKIM and DMARC records to publish.