Start with the data your email platform already has. Export the bounces from your last send and sort them by code. If most read 550 5.1.1 user unknown, you have a list problem. If most read 550 5.7.1, 554 or a 4xx code, receiving servers are pushing back on your sender. Plenty of senders find both, so check each side.
Old data is the first suspect. Addresses decay even when you do nothing: Hunter tracked verified addresses for eight weeks and found 2.3% had turned invalid and another 3.2% had become accept-all (Hunter, 4 November 2025). A list you haven't mailed since last year has been decaying for many of those eight-week stretches.
A fresh import is the second. Event scans, CRM exports and leads from a data provider arrive with dead addresses nobody has tested. When your rate jumps on the first send to a new segment, that import is your cause. Verify every source before it reaches your sending tool; our answer on cleaning a list before cold email gives the order of work.
Catch-all domains hide bounces until after the send. A catch-all server accepts every address at the SMTP step, then bounces the ones that don't exist. About 28% of a 10,000-email B2B benchmark sat on catch-all domains, so expect a share of any business list to land there. Send those rows in a smaller, separate batch.
Blocks sit on the sender side. A 550 5.7.1, a 554 or a run of 4xx replies from one provider points at your domain or IP, and the addresses may be fine. Check SPF, DKIM and DMARC first, then look for a listing: see why a domain gets blacklisted and the 550 5.7.1 blocked fix.
The limits you're measured against are published. Amazon SES recommends a bounce rate under 2%, reviews an account at 5% and may pause sending at 10% (AWS enforcement FAQ, read 24 September 2026). Our page on a good bounce rate sets out the bands, and how to reduce bounce rate covers the fixes. To test a list before the next send, upload it to bulk verification.
Related questions
Why is my email bounce rate suddenly high?
A sudden jump usually follows one event: a new import, a send to a segment you haven't mailed in months, or a new block on your sending domain. Compare the codes against your last normal send. More 550 5.1.1 codes point at the list, and more 5.7.x or 4xx codes point at your sender.
Is a 5% email bounce rate bad?
Yes. Amazon SES puts an account under review at a 5% bounce rate, per its enforcement FAQ read 24 September 2026, and recommends staying under 2%. Pause sends to that list until it's verified.
Can a high bounce rate get my sending account suspended?
It can. Amazon SES states it may pause sending at a 10% bounce rate, and each email platform sets its own limits. Suppressing hard bounces after the first failure and verifying old lists keeps you under those lines.
Should I verify a list that has already bounced?
Yes, verify the rest of it before the next send. The addresses that bounced are only the ones you've mailed so far, and an SMTP check finds the other dead ones without sending anything. ZapBounce charges $5 for 1,000 checks and never bills unknown results.