High bounce rate: clean your mailing list

What it means

A bounce is an email that could not be delivered and came back with an error. There are two kinds:

• Hard bounce: a permanent failure. The address does not exist, the domain does not exist, or the server refuses you for good. Errors start with 5, for example 550 5.1.1 User unknown.
• Soft bounce: a temporary failure. The mailbox is full, the server is busy, or it asks you to try later. Errors start with 4, for example 452 4.2.2 Mailbox full.

The bounce rate is the share of your emails that bounce. Above about 2%, receivers start to worry; above 5%, many email tools pause or close your account.

Why it matters

• Gmail, Outlook and Yahoo see that you send to many dead addresses, which is what spammers with old or bought lists do.
• Your reputation drops, so even your emails to real, happy readers go to spam.
• Old addresses can be turned into spam traps, which can get you blacklisted.

How to check

• Your sending tool: newsletter and transactional tools show the bounce rate for each campaign and the list of bounced addresses. This is the main place to look.
• Your mailbox: if you send from your own mailbox, bounces arrive as "Undelivered Mail Returned to Sender" or "Delivery Status Notification".
• Delivmail: Delivmail cannot see your list, but a test email from the home page (/) confirms that bounces are not caused by your setup, such as a missing SPF, DKIM or reverse DNS. Also check your sending IP and domain at /blacklists.

How to fix it

1. Remove hard bounces immediately. Never send to them again. Most tools do this automatically; check that the option is on and that nobody re-imports old addresses.
2. Watch soft bounces. One soft bounce is normal. If the same address soft-bounces on three to five sends in a row, treat it as a hard bounce and remove it.
3. Remove inactive readers. People who have not opened or clicked anything in 6 to 12 months are risky. Send them one "Do you still want to hear from us?" email, then remove those who do not answer.
4. Use double opt-in. New subscribers confirm their address by clicking a link. Typos and fake addresses never enter your list.
5. Validate forms. Check the address format on your sign-up form, and add a captcha to stop bots.
6. Never buy or rent lists. They are full of dead addresses and traps.
7. Clean old lists before using them. If a list has not been mailed in over a year, a verification service can remove addresses that clearly no longer exist. Then send to the rest slowly, in small batches.
8. Read the error messages. If many bounces mention your IP, your domain or authentication (for example 5.7.x errors), the problem is your setup, not your list.

Check that it is fixed

After cleaning, your next campaigns should show a bounce rate well under 2%. Keep an eye on it after every send, and run a Delivmail test regularly to make sure your authentication and blacklist status stay clean.