All posts

Which List Caused the Bounce Spike? Breaking a Send's Bounce Rate Down by Source

··2 min read

Export the bad send with each recipient's status and where the contact came from, group by that source, and compute hard bounces ÷ rows per group. One group will usually be far above the rest; stop mailing it and verify it before it goes out again.

The bounce rate by source tool does the grouping in your browser and downloads the rows of flagged groups. The 2% flag line is Amazon SES's recommended maximum (sending review FAQ, read 2 November 2026) and Brevo's suspension trigger; the 0.1% complaint line is Google's spam-rate target (email sender guidelines). Code classes follow RFC 3463.

Why the overall rate isn't enough

A 3% campaign bounce rate can be 0.4% on your signup form and 14% on one purchased file. The average tells you there's a problem; the breakdown tells you which list to pause, which vendor to question and which segment to leave out of the next send. It's also what an ESP reviewer wants to hear: "the bounces came from a 2023 trade-show import" is a root cause, "we had bounces" isn't.

Step by step

  1. Export the send. From your ESP or sequencer: one row per recipient with the status (delivered, bounced, complained) and, ideally, the bounce code or reason.
  2. Add a source column if it's missing. Join from your CRM by email; the CSV merge tool does this. Tag, list name, import date or lead vendor all work.
  3. Classify each row. Hard bounce: 5.1.x or 5.2.x, "user unknown", "does not exist". Soft: 4.x.x, mailbox full, deferred. Block: 5.7.x, policy, blocklist, reputation. Complaint: spam report or FBL.
  4. Group and compute. Hard bounce rate = hard bounces ÷ rows in the group. Complaint rate = complaints ÷ rows that didn't bounce.
  5. Flag groups over 2% hard bounces or 0.1% complaints. Mark groups under about 50 rows as too few to judge: one bounce in 20 rows is 5%.
  6. Group by mailbox provider too. If Outlook addresses block and Gmail ones don't, it's your standing with Microsoft, not the list. Bounces from one domain covers that case.

What to do with the flagged group

  • Pull it from every scheduled send and sequence.
  • Download the whole group, not only its bounces, and verify it. The addresses that already bounced are suppressed; the ones next to them in the same file are the likely next bounces.
  • Ask where it came from. If it's a vendor file, compare vendors on their numbers, and test new ones small first: see testing a new lead source.
  • If it was bought, verification won't make it permissioned. See recovering after sending to a purchased list.

Limits

The breakdown reads what your platform recorded; rows with an unreadable status are reported, not guessed. Company domains group as one bucket, since their mail host needs a DNS lookup. It shows which source bounced, not which of its remaining addresses will.

Where SimpleVerifier fits

Upload the flagged group at /verify. Every address gets a live mailbox check, and valid, risky and invalid download separately. The first 100 checks are free with a Google sign-in. Pro is $39 a month with the first 100,000 checks a month at full speed, then slower, never cut off. See pricing.

Common questions

How do I find which list caused high bounces?

Export the campaign's recipients with their delivery status and a column saying where each contact came from, then group by that column and compute hard bounces ÷ rows for each group. The group far above the rest is the cause.

What if the bounces are all at one mailbox provider?

Group by provider (Gmail, Outlook, Yahoo, company domains). If one provider blocks or bounces and the others don't, read the bounce codes: 5.7.x and policy wording mean your sender is being refused, which no list cleaning fixes.

What bounce rate should a lead source have?

Under 2% hard bounces is under every published ESP enforcement line. A source well above that on a first send is usually old, bought or guessed.

Does the breakdown check the addresses?

No. It reads the results your platform already recorded. To check the remaining addresses in a bad group, run them through a verifier.

Check your list before you send

Real mailbox checks over SMTP, and nothing is sent to the address. Sign in with Google and your first 100 email checks are free, no card. Plans for bigger lists are on the pricing page.