What is a bounce and what should you do with it?
A bounce is an email that comes back because the receiving server could not or would not deliver it. There are two kinds: a hard bounce means the address does not exist and must leave your list for good, a soft bounce is temporary (mailbox full, server briefly unreachable) and deserves a few retries. The difference decides what you do with it, and your bounce rate helps decide how all your other mail arrives.
What exactly happens
When your sending server offers a message, the receiving server answers with a code. If it starts with a 2, the message is accepted. If it starts with a 4, there is a temporary problem and you may try again later. If it starts with a 5, it is final: this message will never get in here. Sometimes the answer only comes later, as a separate message from a "mailer-daemon" in the sender's mailbox. Both forms are bounces.
Hard bounce: the address does not exist
A typo at sign-up, an employee who left the company, a domain that no longer exists. The code is typically 550 with text such as "user unknown" or "no such user". Remove that address immediately and permanently. Continuing to send to dead addresses is one of the strongest negative signals there is, because it is exactly what someone with a bought or scraped list does.
An example: a B2B newsletter to 8,000 addresses from the CRM. After three years without clean-up, twelve per cent of the people no longer exist at their old work address. That single send produces almost a thousand hard bounces, and the following week even the sales team's quotes arrive worse at Microsoft addresses.
Soft bounce: temporary
Mailbox full, server temporarily unreachable, message too large, or the recipient applies greylisting and asks you to try again later. The code starts with a 4. A few retries over a couple of hours are normal; a good sending server does that by itself. If the same address keeps soft-bouncing across several sends, treat it as hard after all. A mailbox that has been full for three months is not being read.
The third kind: a block
Not every 5xx code is about the address. Sometimes the receiving server refuses your sender: "blocked", "policy", "reputation" or a reference to a blocklist in the error text. That is not a dead address but a reputation problem, and removing the address solves nothing. If you see many bounces at one receiving provider at the same time, with the same text, this is what is going on. Look at your authentication and your sending behaviour, not at your list.
Why it weighs more than you think
Receiving parties use your bounce rate as a measure of how carefully you handle addresses. A list that produces many bounces depresses delivery of all the messages you send, including those to addresses that are perfectly valid. As a rule of thumb: above two per cent hard bounces per send you need to act; above five per cent you have a problem your next campaign will already feel.
Automate it
Handling bounces by hand works once and then never again. Nobody reads the mailer-daemon messages every morning and updates the list. Make sure hard bounces disappear from your list automatically and soft bounces are counted, so an address is still removed after a few failures.
At 2Mail this happens in two ways. The relay processes bounces automatically (including late ones, via IMAP on the return address) and shows them per domain in the dashboard, so you see which address failed and why. If you build your own software, you also receive every bounce as a signed webhook and can flag the address in your own CRM straight away. How that connection works is described on the email API page; the wider context of bounces, rejections and reputation is on the deliverability page.
What you can do today
- Look at the bounce rate of your last three sends. If it is above two per cent, read the error texts.
- Remove every address with a hard bounce, even if they were "important customers". The address no longer works; the customer probably still exists: ask for a new address.
- Set up a counter for soft bounces: three in a row is enough.
- Send confirmations and invoices through a separate route, so a wave of bounces in your newsletter does not touch your transactional email.
Frequently asked questions
Does an automatic reply ("I am out of office") count as a bounce?
No. The message was delivered; the recipient sent a reply. Do not treat it as an error, although it is useful to filter such replies separately.
May I retry a hard-bounced address later?
Better not. An address that "does not exist" rarely comes back, and every new attempt costs reputation. Ask the customer for a new address through another channel.
What is an acceptable bounce rate?
Below one per cent with a well-maintained list. New double opt-in sign-ups almost never bounce; bounces nearly always come from old data.