Bounce Processing

A bounce is a message returned by the receiving server because it could not be delivered. Bounce processing is what reads those returned messages and removes the bad addresses from your lists.

Without it, MailWizz has nothing to read, so it reports zero bounces. That is not a clean list. It is no measurement at all.

Why zero bounces is a warning sign

A campaign showing 0 bounces and 0 percent bounce rate looks like a healthy list. If no bounce mailbox is connected, it means the opposite: invalid addresses are still on your lists, still being mailed on every send, and nothing is removing them.

This matters more than most people expect. Repeatedly mailing addresses that do not exist is one of the strongest negative signals a mailbox provider can act on. Providers such as Gmail treat a sender who keeps mailing dead addresses as someone who is not managing their list, and they adjust placement accordingly. It is entirely possible to have good content, correct authentication and still sit in the spam folder because of this alone.

If you have imported a list, or it has been some time since it was last cleaned, assume there are invalid addresses on it until bounce data proves otherwise.

Hard bounces and soft bounces

Hard bounces are permanent. The address does not exist, or the domain does not accept mail. These addresses should never be mailed again and MailWizz removes them automatically once bounce processing is connected.

Soft bounces are temporary: a full mailbox, a server that was down, a message rejected for size. MailWizz retries these and only removes the address after repeated failures.

What you need

A dedicated mailbox that receives the bounce messages, reachable over IMAP.

Create it on the mail host for your sending domain, wherever that is. It does not need to be on your MailWizz server, and for most customers it will not be, because your domain’s regular mail is usually hosted elsewhere.

A typical setup:

Mailbox:   bounces@yourdomain.com
IMAP host: mail.yourdomain.com
Port:      993 with SSL, or 143 with STARTTLS

Use a dedicated mailbox, not a personal one. Bounce processing reads and deletes messages from this mailbox as it works through them. Never point it at a mailbox you use for anything else.

Connecting it

You can add the bounce server yourself in the MailWizz interface, or send us the details and we will connect it for you.

To do it yourself:

1. In your customer area, go to Servers, then Bounce servers
2. Choose Create new
3. Enter the IMAP hostname, port, username and password
4. Set the connection security to match your mail host, usually SSL on port 993
5. Save, then use Validate to confirm MailWizz can log in

If you would rather we set it up, open a ticket with the IMAP hostname, username and password, and we will connect and validate it.

Once connected, MailWizz checks the mailbox automatically every few minutes. You do not need to do anything further.

Confirming it works

Bounce data appears after your next campaign, not retroactively. Earlier campaigns keep reporting zero because the bounce messages they generated were never collected.

After your next send, check the campaign’s bounce figures. On a list that has not been cleaned recently, seeing some bounces is the system working correctly. A list that genuinely returns zero bounces across a full campaign is unusual, and worth a second look at whether the mailbox is really connected.

Feedback loops and complaints

Bounce processing handles undeliverable addresses. It does not handle complaints, which are recorded when a recipient clicks “report spam”.

Complaints are more damaging than bounces and are worth watching separately. The most useful source is Google Postmaster Tools, which reports your complaint rate directly from Gmail. Setting it up is covered in Deliverability and warming up.

Keep your complaint rate below 0.1 percent, which is one complaint per thousand delivered messages. Above 0.3 percent, expect placement to deteriorate quickly.

Keeping lists clean

Bounce processing removes addresses that fail after you send. It cannot tell you about bad addresses before you send.

For a list that is old, imported, or of uncertain quality, run it through an email verification service before the first campaign. Removing invalid addresses in advance is considerably better than discovering them by mailing them, because by then the damage to your reputation is already done.