Mailshake Emails Bouncing: What to Do in the Next Hour

If Mailshake emails are bouncing, turn the campaign's Running toggle off now, export the bounces, and read the bounce messages before sending anything else. Bounces that say the mailbox does not exist mean your list is the problem. Bounces that mention policy, spam or reputation mean your sending setup is the problem, and more volume makes it worse. Then verify every prospect that has not been emailed yet. SimpleVerifier Pro does that for $39 a month: the first 100,000 checks a month at full speed, then slower, never cut off, with 100 free checks to try first.

Sources for the Mailshake behaviour below are Mailshake's own help pages, read on 9 November 2026. Tools change, so confirm anything that matters in your account.

What Mailshake does on a bounce, and what it does not

Mailshake's FAQ says that when an email bounces, Mailshake clears out any pending messages for that recipient, so the effect is the same as pausing them. That protects you from emailing the same dead address again in step two.

The same FAQ says Mailshake never pauses recipients on its own and that pausing is a function for you and your team. We also looked for a help article describing an automatic campaign pause when the overall bounce rate crosses a percentage, and did not find one. That matters. Some sending tools stop a campaign at a threshold; from its help pages, Mailshake does not appear to. If you are watching a rising bounce number, nothing is going to brake for you.

Mailshake's own pages do describe two other kinds of stop:

• Provider quotas. The FAQ says that if Google's limits are hit, Mailshake stops sending and resumes after about 24 hours. Another help article says Mailshake detects when a quota was hit and pauses sending for a period before resuming. That is a volume limit, not a verdict on your list.
• Abuse monitoring. Mailshake's help says that if its monitoring flags your account's sending history you get an email warning, and if your team is flagged a second time Mailshake blocks your sending. A list that bounces heavily is exactly the kind of sending history that gets noticed, so a first warning is a signal to fix the list, not to wait it out.

What to do in the next hour

1. Pause the campaign. On the Campaigns page, switch the Running toggle off. Mailshake's help says a grey toggle means paused and that switching it back on resumes where it left off, so you lose nothing by stopping.
2. Export the bounces. Pull every bounced recipient with the bounce message. You need the text, not just the count.
3. Sort by message. Two piles. In pile one are messages like "user unknown", "mailbox not found" or "address rejected": the address is bad. In pile two are messages mentioning policy, blocklists, spam content or reputation: the receiving server is refusing you, not the address. The bounce message analyzer and our guide to provider bounce messages help you read them.
4. Note which domains bounced. If most of the failures sit on a handful of recipient domains, paste the list into the bounce domain hotspot finder. One bad domain is a different fix from a bad list.
5. Do not resume yet. Everything unsent is still a list you have not checked.

Why Mailshake emails bounce

Most causes come down to the data and a few to the sending side.

• The addresses were never checked. Prospect data from scrapers, exports and old spreadsheets goes stale. People change jobs and mailboxes get closed. See why cold emails bounce.
• Catch-all domains accepted the mail and bounced it later. Some servers accept everything at the door and reject afterwards, so an address looks fine until the bounce arrives. See catch-all leads in sequences.
• A typo or a changed domain. One wrong character in an import column bounces every time.
• The sending mailbox is the issue. If the bounce text names your domain, your authentication or your reputation, the list is not the cause. Our post on telling a list problem from a domain problem walks through the split.
• A provider limit. Quota pauses are not bounces, though they can look like a sudden stop.

Step-by-step fix

1. Delete the hard bounces and suppress them so a future import cannot add them back. See cold email suppression lists.
2. Treat repeated soft bounces carefully. A full mailbox or a temporary server problem can clear; the same address failing over several sends should be treated as dead. The hard versus soft bounce post explains the call.
3. Verify the rest of the prospects. Export the prospects not yet emailed and upload the file to the verify page or the CSV email verifier. You get valid, risky and invalid with a reason.
4. Rebuild the campaign from valid only. Put risky and catch-all addresses in a separate, smaller campaign you can pause on its own. Use the catch-all send limit calculator to size that batch.
5. Restart small. Send a fraction of the usual volume for a day or two and look at the bounce count before scaling. If bounce messages were policy blocks, follow restarting sending after a block instead of sending the same volume again.

For the full triage, what to do after a high bounce rate covers the first 24 hours.

Prevention: verify before every send

Because Mailshake leaves the stop decision to you, the cheapest control is upstream. Verify each new prospect file before it is imported, and keep a bounce line you will pause at before you launch, not after. Our post on pausing a campaign at a bounce spike shows how to set that line, and the daily bounce rate tracker lets you log each day's send and bounces so a rising trend is visible early. For the import step specifically, see verifying Mailshake prospects.

Mailshake's own bounce guidance on its blog is a general benchmark, not a promise about your domain. Your recipients' mail servers decide, so the numbers that count are the ones in your own sending history.

A worked example of the triage

Say a campaign has gone out to a few hundred prospects and the bounce count has crept up over two days. You switch the Running toggle off and export the bounces. Reading the messages, most say the mailbox could not be found, spread across many different companies. That is the list. A handful instead say the message was blocked for policy reasons, all from the same recipient provider. That is a separate problem on the sending side, and it needs its own handling.

The mistake is to treat both piles the same. Deleting the dead addresses and resuming does nothing for the policy blocks, which will keep happening at the same volume. Slowing down for the policy blocks does nothing for the dead addresses, which will keep bouncing at any volume. Two causes, two fixes, and the messages tell you which pile each bounce belongs in.

What not to do

• Do not turn the campaign straight back on because Mailshake cleared the pending messages for bounced recipients. That only covers the people who already bounced, not the rest of the list.
• Do not wait for a second warning. Mailshake's help describes a warning on the first flag and a block on the second. Use the warning.
• Do not buy more prospect data to replace the bounced ones before checking the new data. A fresh export has the same problem as the old one.
• Do not delete the evidence. Keep the bounce export so you can compare sources later.

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. You can try 100 free checks first, no card. That covers a full prospect file for most campaigns, and you can re-verify each new file before it goes into Mailshake. See pricing.

Bottom line

Mailshake clears pending messages for a bounced address but, from its help pages, does not stop a campaign at a bounce rate for you. Pause it yourself, read the messages, remove the dead addresses and verify the rest before you turn the toggle back on. Try the 100 free checks on the unsent prospects first and see how many would have bounced.