List decay
The steady loss of accuracy in a mailing list as people change jobs and abandon addresses, turning valid addresses into bounces and, later, recycled spam traps.
What it means
Spamhaus puts it simply: addresses are constantly abandoned or re-used, so lists need to be kept current. Abandoned addresses first bounce; some are later converted into recycled spam traps, after a period of inactivity for which M3AAWG suggests 12 months as a minimum.
M3AAWG counts a list not mailed for a year or more among the marks of a dirty mailing list, and Spamhaus warns that stagnant lists provoke high complaint rates when reactivated, even when every address was confirmed.
Decay is why lists need ongoing bounce processing, sunsetting and occasional reconfirmation rather than a one-time clean.
Related terms
On this site
Sources
What this definition rests on. The wording is ours; the facts are theirs.
All 432 terms in the glossaryThe wiki: blocklists and bounce codes