TL;DR
- Lists decay because people change jobs, companies change domains, and personal addresses get abandoned.
- The one figure we can source: Smartlead cites 25-30% annual data decay. Treat it as a vendor estimate, not a law.
- Measure your own: verify a random 200-1,000 address sample and track the undeliverable share over time.
Why lists decay
Decay is the share of addresses that worked when you collected them and no longer reach anyone. The causes are the same everywhere:
- Job changes. A work address belongs to an employer. When the person leaves, the mailbox is usually closed, and mail to it starts to bounce.
- Company changes. Acquisitions, rebrands and shutdowns move or kill whole domains at once.
- Domain and provider changes. A company switches mail systems or tightens its filtering. Addresses that were accepted may start rejecting, or the domain may start accepting everything (a catch-all).
- Abandoned personal addresses and old signups that nobody reads.
- Bad data from day one: typos, guessed address patterns and throwaway addresses.
What the numbers say
Smartlead's bounce-rate post (source, checked 2026-10-17) cites 25-30% annual data decay. That is a sender-platform estimate, not a measurement we can reproduce. Verifiers and data vendors publish other figures, some far higher, built on different definitions (all contacts or only email, B2B or consumer, ageing from when). We don't repeat numbers we cannot trace to a method, so we leave them out. The honest answer is that your decay depends on who is on your list, how old the data is and where it came from.
Decay also shows inside a campaign. Senders report bounces rising on later sequence steps as a list ages, which is why re-verifying before follow-ups is part of the workflow.
Measure your own decay
- Take a random sample of 200-1,000 addresses from the list, not the first rows.
- Verify it in Mail Exchange. The 3-day free trial covers a small sample.
- Record the share undeliverable, plus the share risky (catch-all) and unknown.
- Repeat on a new sample from the same list every quarter, or sample lists of different ages and compare.
The undeliverable share on a sample is a good estimate for the whole list. It also tells you whether the full list is worth verifying, as an ongoing routine.
What to do about it
- Verify before every large send, and before the later steps of long sequences.
- Keep a "last verified" date next to each address in your CRM.
- Remove undeliverable addresses promptly; separate catch-alls (how).
- Stay under the bounce levels that damage a domain.