Bounce explainer

Bounces from any mail server. It reads the delivery status fields (RFC 3464), the basic SMTP reply codes such as 550 (RFC 5321), every enhanced status code in the IANA registry such as 5.1.1 (RFC 3463 and later), and the known wording of Gmail, Microsoft 365 and Outlook.com, Yahoo and AOL, plus the Spamhaus, Barracuda, SpamCop and Proofpoint blocklists.

A hard bounce means the address can’t receive mail: the mailbox or its domain doesn’t exist, or the account is disabled. Stop sending to it. A soft bounce is temporary, such as a full mailbox, greylisting or rate limiting; the sending server retries for a while. A refusal is a third kind: the address may be fine, but the receiving server refused this message, for example because authentication failed or the sending IP is on a blocklist.

550 is the basic reply code: 2 means delivered, 4 temporary, 5 permanent. 5.7.26 is the enhanced code: the same first digit, then the area (1 address, 2 mailbox, 3 mail system, 4 network, 5 protocol, 6 content, 7 security or policy), then the detail. Providers also use their own details, such as Microsoft’s 5.7.509; the explainer says when a code isn’t in the standard registry.

Not unless you ask for the AI explanation. The explainer runs in your browser, and the bounce is never put in the page link or saved. Explain with AI sends only the error part to tanase.ai, which passes it to Anthropic’s API (Claude): the original message is cut off first and the name before every @ is replaced, and the page shows exactly what will be sent. tanase.ai keeps neither the text nor the answer; Anthropic handles it under its API terms.

It is given the codes and their standard meanings along with the text, and told to stay with what it is sure of, but AI text can be wrong. The codes, their meanings and the steps above it come from the standards and from the providers’ own documentation.

Because the receiving server refused the message, not the address. Common causes: SPF, DKIM or DMARC failing for the sender’s domain, the sending IP address on a blocklist, a poor sending reputation, missing reverse DNS, content or attachments a filter blocks, or a message over the size limit. The explainer names the cause and the tool to check it with.

Remove hard bounces after the first one: mail providers watch how often a sender hits addresses that don’t exist. For soft bounces, many senders remove an address after several in a row over a few weeks. Validating a list before sending keeps bounces down in the first place.