How to Find Which Domains Are Causing Your Email Bounces

To find which domains cause your bounces, group the send by recipient domain and compare how many addresses you sent to with how many bounced at each one. Your overall bounce rate is one number. The domain table behind it is where the action is: often a handful of company domains hold a disproportionate share, and the rest of the list is fine.

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Why an overall rate is not enough

The bounce rate calculator tells you how many of your sends bounced. Bounce rate by source tells you which list or data vendor they came from. Neither tells you which recipient domains are involved, and that is what you need if you want to act on specific addresses instead of a whole list.

Knowing the domain changes what you do next. If the bounces come from one company, you are not facing a list quality problem, you are facing a company problem: it closed, merged, changed its mail system or blocks you. If the bounces spread evenly over hundreds of domains, the list itself is the problem and the answer is verification of the whole file.

What you need to run the check

An export from your sequencer or email platform that has, for each row, the email address and the status. The status column can say anything, as long as the bounced rows contain a recognizable word. The tool looks for the word "bounce" by default and lets you change it if your platform uses different text, in any letter case. If an address appears more than once, the repeats are merged and the address counts as bounced if any of its rows is a bounce.

Everything runs in your browser; the export is not uploaded to SimpleVerifier.

How to run it

1. Export the send or lead activity from your sequencer or email platform, with the address and the status of each row.
2. Upload it (.xlsx or CSV) or paste the cells, and set the status text that marks a bounce if yours is not the word bounce.
3. Read the flagged domains. Set the bounce count and share that make a domain a hotspot, and download the table.
4. Check the addresses that have not bounced at the flagged domains before you email them again.

What counts as a hotspot

You set the thresholds. A domain is flagged when at least your chosen number of its addresses bounced and the bounced share is at least your chosen percent. Two special rules apply:

• A domain where every address bounced, with two or more sent, is flagged on its own.
• Domains with a couple of bounces below your thresholds are marked to watch.

Personal mailboxes such as Gmail and Outlook are counted together and never flagged. A bounce there is one person's mailbox, not a company's mail setup, so it says nothing about the next address at the same provider.

Why bounces cluster on one domain

Common causes:

1. The company closed or moved its mail. Every address at the domain fails because the mail setup no longer exists.
2. You guessed a naming pattern wrong. If you built addresses from first and last names with a pattern, and the pattern is wrong for that company, every address you built bounces together.
3. A mail server rejects your sender. The server may refuse your sending domain or IP. The recipients are real; you are what is blocked.
4. One outdated source filled the domain. A stale data pull holds many addresses at the same company, all of which have since left.

The text of the bounce tells these apart. A message saying the mailbox does not exist points to causes 1, 2 or 4. A message about policy, reputation or blocking points to cause 3. Enhanced status codes give a structured way to read it: see our glossary on enhanced status codes and RFC 3463. An every-address bounce does not prove the domain is dead; read a few bounce messages before you drop it.

For a deeper walk-through of one domain, see bounces from one domain.

What to do with the rest of the addresses

The addresses at a hotspot that have not bounced yet are the interesting ones. The tool lists them for exactly this reason. Do not send to them again until they have been checked. Mail servers can accept a message and report the failure afterwards, as described in RFC 5321, so a quiet address at a bad domain is not proof that it works. It may just not have bounced yet.

Use the check button under the result, or upload those addresses at /verify. The results sort into valid, risky and invalid; keep the valid ones and drop the invalid. If the domain is a catch-all, a mailbox check cannot confirm individual addresses, and what to do with catch-all emails covers the options.

If the cause is that a server rejects your sender, verification will not help: the address may be real. That is a deliverability problem to fix on your side, and the bounce budget per inbox is the place to start deciding how many such failures you can afford.

Reading the table

The output has one row per domain: sent, bounced, rate, and a flag. A few ways to use it:

• Sort by bounced count to find the domains costing you the most.
• Sort by rate to find small domains where everything fails, which often points to a wrong naming pattern.
• Look at the sources behind the flagged domains. If many of them came from one export, the export is stale. Bounce rate by source confirms it.
• Pull the flagged domains out of the next list before you send.

Limits

• It trusts the status column. A wrong or missing status gives wrong counts.
• It cannot tell a closed domain from one that rejects your sender; the bounce message does.
• Domains are matched as written, so a company with several domains shows as several.
• Soft and hard bounces count the same unless your status text tells them apart. For repeated temporary failures, use the repeat soft bounce finder, and to pull the bounced addresses into their own list, the bounced email extractor.

Prevention beats diagnosis

Finding a hotspot after a send means the bounces have already happened. The way to avoid the next one is to verify the list before it goes out, so the dead addresses at a closed company are removed on day one. How often to repeat that is covered in how often to verify your list.

SimpleVerifier Pro is $39 a month, with the first 100,000 checks a month at full speed, then slower, never cut off. See pricing.

Verify the addresses before the next send →