What Is a Hard Bounce?

A hard bounce is a permanent email delivery failure: the receiving server rejects the message and tells you it will never accept it at that address.

The SMTP standard, RFC 5321 (September 2026), defines this as a 5yz reply code, a "permanent negative completion". The enhanced codes in RFC 3463 (September 2026) give the reason. 5.1.1 means the mailbox does not exist. 5.1.2 means the domain does not exist.

A soft bounce is the opposite: a temporary 4yz failure such as a full inbox. The server may accept the same message later.

Why it matters for outbound

Mailbox providers watch your bounce rate. A few hard bounces on a cold send tell them you are mailing addresses you never checked. Keep hard bounces as close to zero as you can. The more you send, the more your sending domain carries the cost.

The cause on a B2B list is usually decay. People change jobs, companies change domains, and an address that was good last year is gone. See why emails bounce for the full list of causes.

Never retry a hard bounce. Suppress the address after the first one. For the rate maths, read what a bounce rate is and how to lower your email bounce rate.

Example

You send 500 emails from your sending domain. The log comes back with this:

550 5.1.1 <jane.doe@acme.com>: Recipient address rejected: User unknown

Jane left Acme in March and IT deleted her mailbox. The reply starts with 5, so it is permanent. Your tool should add jane.doe@acme.com to a suppression list and never send to it again.

If 6 of the 500 sends bounce this way, your hard bounce rate is 1.2%. Verify the rest of the list before the next send.

In LeadOcean data

LeadOcean holds an email_status on every person record, with 13 values (openapi.json, September 2026). The docs call verified and catch_all_valid safe to send. catch_all cannot be confirmed either way, because the domain accepts everything. The count_leads tool counts all three by default.

The docs say do not send to invalid, disposable, spam_trap or abuse, and use risky, role and derived with care. unknown and untested are not checked clean either.

To build a list, pass emailStatus to /v1/people/search or the search_leads MCP tool. In an export, request email_N_status next to email_N_address through POST /v1/exports. Plans are on pricing.

LeadOcean does not refund or credit bounced emails. Read email_status before you send. Statuses are a check at a point in time, so a verified address can still bounce later.

Related terms

• SMTP verification
• Domain reputation
• Role-based email address