A bounce is an email that couldn't be delivered. Every sender has some – what matters is how many and how you react. Mailbox providers read a high bounce rate as a sign of neglected or bought lists and lower your reputation. This guide explains the kinds of bounces, the key SMTP codes and what to do about them.
Hard and soft bounces
- Hard bounce: permanently undeliverable. The mailbox doesn't exist, the domain has no mail server or the address is invalid. Suppress such addresses immediately – further attempts only hurt.
- Soft bounce: temporarily undeliverable, for example because the receiving server is overloaded, the mailbox is full or the server accepts only later (greylisting). Retrying with increasing intervals helps.
- Policy rejection: the server refuses the message because of its content or the sender's reputation. The mailbox exists; suppressing it would be wrong. Check authentication, content or sending behavior instead.
Reading SMTP codes
Receiving servers reply with a three-digit code and often an enhanced status code per RFC 3463:
| Code | Meaning | Reaction |
|---|---|---|
250 | accepted | delivered |
421, 451 | temporary, e.g. throttling or greylisting | retry later |
452 4.2.2 | mailbox full | retry, give up after days |
550 5.1.1 | recipient unknown | suppress immediately |
550 5.1.10 | domain accepts no mail (null MX) | suppress immediately |
550 5.7.1 | policy, spam or reputation | don't suppress, investigate |
550 5.7.26 | authentication missing (e.g. Gmail) | check SPF, DKIM, DMARC |
Not every bounce is immediate. Some servers accept a message first and later send a delivery status notification (DSN) to the return path. These asynchronous bounces must be processed too.
What bounce rate is fine?
Rule of thumb: below 2% is healthy, above 5% is a serious warning sign. A freshly imported old list can easily reach 10% or more – don't send to it at full volume.
Common causes
- Outdated lists: a noticeable share of business addresses expires every year as people change jobs.
- Typos at signup:
gmial.com,hotmial.com. - Bought or scraped addresses, often including spam traps.
- Long sending gaps after which many addresses no longer exist.
How to lower the bounce rate
- Double opt-in for every signup – it filters out typos and fake addresses automatically.
- Suppress hard bounces immediately and never email them again.
- Sunset inactive recipients: after months without engagement, send a last check-in, then remove them.
- Reactivate old lists carefully: in small batches, asking for reconfirmation.
- Check authentication when policy rejections show up.
5.1.x responses put the address on the suppression list at once; ridu never sends there again. Temporary errors are retried with growing intervals for up to 72 hours. Policy rejections don't suppress addresses but are visible in the dashboard. Asynchronous bounces come back via the return path and are processed. If an organization's bounce rate exceeds 4%, ridu pauses sending before reputation suffers.