How to Clean a Campaign Monitor List: Verify Before You Send

To clean a Campaign Monitor list, export your subscribers, verify the CSV, remove the invalid addresses and put risky ones in their own segment before the next send. Campaign Monitor suppresses hard bounces after they happen, but it cannot know an address is dead until a message fails. SimpleVerifier Pro checks the list first for $39 a month: the first 100,000 checks a month at full speed, then slower, never cut off, with 100 free checks to start.

The Campaign Monitor behaviour in this post comes from its own help article on the bounced emails report, read on 10 November 2026. Confirm anything critical in your own account, since details can change.

What Campaign Monitor does with bounces

The help article describes it this way:

• Hard bounce: a permanent delivery failure. The address is removed from your subscriber list and added to your suppression list, which stops it being accidentally re-imported. The article warns that moving such addresses back to active status hurts your deliverability.
• Soft bounce: a temporary failure. The address was recognised but the message was not delivered. These are not treated as invalid, and Campaign Monitor tries to send the next campaign to them.
• Retries: for soft bounces it retries after five minutes, then about every 15 minutes, and stops after 14 hours. Hard bounces are not retried.
• Conversion: an address becomes a hard bounce after 5 soft bounces with no opens or clicks in that period, and any later interaction resets the soft bounce count to zero.
• Where to look: Campaigns, then a campaign in the Sent category, then Bounce Summary in the left sidebar.

The gap this leaves

The suppression list is a good safeguard. It means the same dead address is not mailed twice. But think about the timeline for an old list. Every dead address costs one hard bounce before it is suppressed. A list of ten thousand contacts imported from a spreadsheet will announce its bad addresses to mailbox providers in a single campaign. Soft-bounce handling is slower still, since five soft bounces with no opens or clicks must happen first.

You want the bounces to happen in a verifier, not in a live send. Read why cold and bulk emails bounce for the common causes, and hard bounce versus soft bounce for how to read them.

Step by step: clean a Campaign Monitor list

1. Review what has already bounced

Open the Bounce Summary of your last few campaigns. Look for patterns: one recipient domain, one import batch, one signup source. Paste the domains into the bounce domain hotspot finder if you have a long list.

2. Export the subscribers

Export the list or the segment you plan to send to. Keep the original file safe and work on a copy.

3. Remove duplicates and fix columns

Use the remove duplicate emails tool so you only verify each address once, and the CSV column editor to drop columns the verifier does not need. See fixing broken CSV rows if the export looks off.

4. Verify

Upload the file to the verify page or the CSV email verifier. You get valid, risky or invalid for each address with a reason. Email verification statuses explained covers what each one means.

5. Decide what to keep

• Invalid: remove. They will bounce.
• Risky or catch-all: keep but segment. These are mailboxes the server would not confirm. Send to them separately and watch the bounce count. See catch-all emails and what to do.
• Valid: your main segment.

6. Update Campaign Monitor

Import the valid file back as your sending list and the risky file as its own segment or list. Upload the invalid addresses to your suppression list so a future import cannot bring them back; Campaign Monitor's help says the suppression list exists for exactly this purpose. If you are bringing a list in from another tool, the Campaign Monitor list import guide covers the file preparation.

7. Send and compare

After the next campaign, open the Bounce Summary and compare. If the number is still higher than you expect, read the failure messages with the bounce message analyzer. Some messages point at sender reputation, not the list.

Why this matters for reputation

Your bounce rate is one of the clearest signals mailbox providers see. A sender whose bounces climb looks careless or like they bought a list. The effect lasts beyond one campaign, because the reputation attaches to your sending domain and IP. Our post does email verification hurt sender reputation explains why checking in advance does not, and the email deliverability checklist puts list hygiene in context with authentication and content.

Common mistakes

• Relying only on the suppression list. It protects against repeats, not the first bounce.
• Re-activating bounced addresses. Campaign Monitor's help warns this hurts deliverability.
• Importing a list and sending the same day. Verify first.
• Treating silence as invalid. An address that never opens may have a working mailbox. Use a sunset policy for that.
• Deleting all risky addresses. Segment them instead.

How often

Before big sends, after every import and on a regular schedule. The list decay calculator estimates how much of a list goes stale over time, and the list cleaning frequency calculator turns that into a schedule.

A worked example

Imagine an agency that manages a Campaign Monitor account for a client with a list of several thousand customers, collected over years at a shop counter, through a website form and from an old newsletter tool. The Bounce Summary of the last campaign shows a few hard bounces, which Campaign Monitor has already suppressed. That looks healthy, but it only counts the addresses that have failed so far.

The agency exports the list and verifies it. Counter-collected addresses show a clear typo problem: misspelled domains and missing letters. The old newsletter tool's addresses show many invalid mailboxes, because those people left years ago. The website form addresses are mostly fine. The agency sends the typo list back to the shop to correct, uploads the clearly dead addresses to the suppression list, and mails the valid group first.

Because the dead addresses were removed before the send, the campaign's Bounce Summary stays quiet, and the client's domain avoids a spike that could have slowed future campaigns. If you do this work for clients, the agency client list verification workflow shows how to document it.

Questions to ask before each import

• Where did these addresses come from?
• Have I compared them against the suppression list?
• Do I have a record of the invalid file outside Campaign Monitor?
• Should the risky addresses be a separate list?

What it costs

SimpleVerifier Pro is $39 a month. The first 100,000 checks a month run at full speed, then they run slower, and you are never cut off. That covers a full Campaign Monitor export and each new import afterwards. Try 100 free checks on a slice of your own list first, no card. See the pricing page.

Bottom line

Campaign Monitor suppresses hard bounces and converts repeated soft bounces, both after the fact. Export, verify, split by result and re-import, and your first bounce becomes a line in a report instead of a send that damaged your reputation.