Lists decay for boring reasons: people change jobs (work addresses die with the badge), abandon inboxes, and outgrow interests. Industry estimates cluster around 2–2.5% per month.
The compounding
At 2.2% monthly decay, a 10,000-address list has ~7,650 live addresses after a year and ~5,860 after two. Your subscriber counter still says 10,000. Every metric computed against that denominator — open rate, click rate, revenue per subscriber — is quietly wrong, and getting wronger.
The asymmetry
Dead addresses don't just dilute stats. Mailbox providers read sustained bounces as list-buying behaviour, so the dead part of your list actively punishes delivery to the living part. Decay is not neutral; it's corrosive.
The calendar
- Monthly: validate new additions (they decay fastest — typos and burners die immediately)
- Quarterly: full-list validation pass
- Semi-annually: dormant review — re-permission or suppress people with zero opens in 6 months
- Before any major send: validate everything you haven't checked in 90 days
The work is minutes per month with tooling, and existential without it. That's the whole pitch for continuous validation — yours truly or anyone else's.
Run this on your own list.