Model the list
Decay covers job changes, company closures, address changes and mailbox retirements. 2–3% a month is the range most B2B data vendors publish; consumer lists decay slower. Domain replacement cost is registration plus three inboxes plus three weeks of warmup you cannot send during.
How it works
Decay
Live contacts after n months = size × (1 − rate)ⁿ. The rate covers job changes, closures and retired mailboxes; 2–3% a month is what B2B vendors publish.
Bounce
Every dead contact is a hard bounce if sent to unverified. The rate is compared with the 2% line Gmail and Microsoft treat as careless and the 5% line that gets fresh domains throttled.
Cost
Wasted sends are dead contacts times sequence steps. Verifying costs per-contact; not verifying risks a domain, priced at registration plus inboxes plus three weeks of lost sending.
Replace
Each month, the contacts that died are what you must source to hold the list at size. That is the recurring data bill.
Where the 2–3% comes from
Roughly a quarter to a third of B2B contacts change in a year: they move jobs, get promoted into a role you are not selling to, their company is acquired and the domain retired, or the mailbox is simply closed. Data vendors who track this publish figures between 22% and 35% a year, which is 2–3% a month compounded. Consumer lists are slower because people keep personal addresses for a decade. The rate you pick matters less than the shape of the curve: even at 2%, a list is over the 2% dead line in the first month.
Why that line is the one to watch
Gmail's and Microsoft's bulk-sender guidance treat a hard-bounce rate above about 2% as a sender who does not verify, and they weight it against the domain, not the campaign. A fresh cold email domain — three inboxes, warmed for three weeks — can be throttled after one send at 5% bounces. So the question is never "is the list good" but "when was it last verified", and the model shows the answer is almost always "too long ago". A list verified in January is over the line by March.
The cost comparison
Verification costs a fraction of a cent per address. A burned domain costs the registration, the inboxes, and — the expensive part — three weeks during which those inboxes cannot send while replacements warm up. At the defaults, verifying a five-thousand-contact list costs about the same as a coffee, and losing one sending domain costs about forty of them. That ratio is why every list I send to is verified within days of the send, not weeks, regardless of where the data came from.
Replacement is the recurring bill
The month-by-month table shows the part that gets forgotten in budgeting: holding a list at five thousand live contacts means sourcing a hundred and twenty-five new ones every month, forever. Over a year that is a third of the list again. Teams that budget once for the data and then wonder why reply rates fell in month eight are looking at this table without knowing it. Verify before every send, replace what died, and the cost per live contact stays close to what you paid.
Frequently asked questions
What decay rate should I use?
2.5% a month for mid-market B2B is a reasonable default. Enterprise lists with senior titles churn faster, around 3%; SMB owner-operator lists slower, around 2%; consumer lists around 1–1.5%. If you have bounce data from a past send, back into the rate from that.
Does every dead contact bounce?
Most do. Some addresses are kept alive as catch-all or forwarded to a shared inbox, which is worse — they do not bounce and nobody reads them. The model treats dead as bounce, which is the conservative case for domain risk.
Why is the 2% line crossed so quickly?
Because 2% is a small number and decay compounds from day one. At 2.5% a month, a perfectly clean list is 2% dead after 24 days. That is the practical case for verifying per campaign rather than per quarter.
What is included in the domain replacement cost?
Registration, three mailbox subscriptions for a month, and the three weeks of warmup during which the new domain cannot carry volume. Labour is not included, so the real figure is higher.
Does verification stop decay?
No. It removes what has already died so you do not send to it. The list keeps decaying at the same rate; verification just resets the bounce rate to near zero on the day it runs.
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