A bounce is an email that never reached the inbox. A cold email bounce rate is too high when you see more than a handful of bounces in a hundred sends, and the sensible response is to pause, find out why, and fix the list before you send another message. A fixed percentage from a blog post is less useful than a rule you write down and follow.

Hank sells replacement windows and mailed a list of 200 local contractors he bought from a directory. Fourteen emails bounced back within an hour. That is 7 in 100. This piece uses his list to show what to check, what to fix, and the rule he adopted afterward.

Hard bounces and soft bounces

A hard bounce is permanent. The address doesn't exist, the domain is dead, or the server has refused your mail for good. In the bounce message you will usually see a code starting with 5, such as 550 5.1.1, with wording like "the email account that you tried to reach does not exist." Remove these addresses at once.

A soft bounce is temporary. The mailbox is full, the server was down, or the message was too large. These codes usually start with 4, though some full-mailbox errors use a 5 code. A soft bounce is worth one retry a day or so later. If the same address soft-bounces again, treat it as dead.

One more case looks like a bounce but isn't about the address: a block. The receiving server refuses your message because of your reputation, your content or your authentication. Those messages tend to mention policy or spam, and codes starting 5.7 point that way. We'll come back to telling the two apart below.

Why bounces hurt your sending

A mailbox provider watches what happens when you send. An address that doesn't exist is a signal that you don't know who you're writing to, which is how mass mailers and scrapers behave. A few bounces are normal. A steady stream of them reads as a list nobody checked. The longer the pattern runs, the more your future mail is judged by it.

Bounces also waste your daily volume. If you cap yourself at 30 sends a day (see how many cold emails per day from Gmail) and 7 of them go to dead addresses, you've used a fifth of the day on mail that could never be read. For the mechanics of reputation, why cold email lands in spam, mechanically is the longer read.

A rule for when to pause

There's no official number that applies to everyone, and you should distrust anyone who cites one with great confidence. Here is a rule of thumb that is easy to follow and errs on the cautious side. Treat it as editorial advice, not a limit set by Google.

  • Check after every batch of 25 to 50 sends, not at the end of the month.
  • One or two hard bounces in a batch: remove them, look at where those addresses came from, and continue.
  • More than about 5 percent hard bounces in a batch, which is 2 or 3 in a batch of 50: stop sending from that list until you've verified it.
  • More than about 10 percent: stop for the day and treat the whole list as unreliable.

Hank's 14 out of 200 was 7 percent. By this rule he should have paused after the first batch of 50, where he'd have seen about 3 or 4 bounces, and checked the list before sending the other 150. He didn't, and that is the lesson.

Verify addresses before you send

Verification means checking that an address looks real before you mail it. Checks range from simple (is the domain valid, does it receive mail) to deeper (does the mail server say the mailbox exists). Many email verification services do this for a small fee per address, and some lead databases verify as part of their data.

No verifier is perfect. Some mail servers accept anything and reject later, so a verifier can't tell for sure. Treat a "valid" result as likely, not certain. What you can do is test a small sample first. Verify and send to 20 addresses, and see what comes back. If you need a way to evaluate a data source, lead data you can check before paying goes through how to test one.

Clean the list after every send

After each batch, do three things. Remove hard bounces from the list permanently, so they can't reappear when you import a fresh file. Move soft bounces to a retry list and remove them if they bounce again. And add anyone who asked to opt out to a do-not-contact list you keep forever.

A spreadsheet is enough: one tab for the live list, one for removed addresses with the reason. Before every new upload, compare it against the removed tab. Bounced addresses that get re-added from a stale export are one of the most common ways to keep damaging your reputation.

Common causes: old data and guessed addresses

When Hank looked at his 14 bounces, the pattern was clear. Nine were people who had left their companies. Three were typos in the directory. Two were guessed. He'd tried first.last@ for a few contractors, and half the guesses were wrong.

Old data is the biggest cause. People change jobs and businesses close, and a list loses a meaningful share of its value every year. A list bought long ago is the riskiest of all; read the list is not the customer for why a big list is not a good one.

Guessed addresses are the second cause. Guessing a pattern (first.last@company.example) and sending can work, but it produces bounces, and some of the "valid" guesses will go to a person you did not intend. Verify before sending. If you're trying to reach an owner, how to cold email the owner instead of the gatekeeper covers finding a name and checking the address.

How to tell a bounce from a spam block

Read the message. A bounce for a bad address says the user is unknown or the account doesn't exist. A block says something like your message "was blocked", "looks like spam", "was rejected due to policy", or complains about authentication. If many different addresses at different companies all reject you with block-style messages, the problem is you, not them. Check your authentication first (the walkthrough is in SPF, DKIM and DMARC for cold email), then slow down and warm up.

If a few addresses at one company reject you and the rest don't, it may be that company's own filter, which you can't fix. Move on.

Rocketship deals with a lot of this for you. Its AI worker removes a hard-bounced address instead of sending to it again, and enrichment only charges for a verified contact when one is found. Because mail goes out through your own Gmail, a bounce lands where the worker can see it and act. You can read more on the AI workers page.

A clean list does more for delivery than any wording trick, and keeping it clean is mostly bookkeeping. Rocketship finds the buyers, emails them from your own Gmail, follows up, and books the meeting on its own. Plans that get you customers start at $24.99 a month (Launch), and every monthly plan starts with a 7-day free trial, card required.