What happens to bounce data over time (aging)?
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An email address that bounced two years ago might work fine today. Or it might have become a spam trap. Bounce data ages, and acting on old data without thinking it through can get you into trouble either way.
Soft bounce records have a shorter useful lifespan. A mailbox that was full last quarter might have been cleaned out. A server that was temporarily down might be running normally. For soft bounces, ESPs typically clear the failure record after successful delivery, or start retrying after a cooling period. If you're managing your own suppression rules, don't permanently suppress soft bouncers without a clear threshold (like, five consecutive fails over 30 days).
Hard bounce records are different. A hard bounce means the address was invalid at the time of the bounce. That doesn't always mean it's permanently invalid. Email addresses do get recycled, especially at free providers like Gmail or Outlook, where abandoned accounts get reassigned to new users. An address you hard-bounced three years ago might now belong to a completely different person.
This recycling is actually what creates recycled spam traps: formerly valid addresses now used by mailbox providers to catch senders mailing stale data. If you hard-bounced an address in 2021 and mailed it again in 2024, you might be mailing a trap. The practical rule: don't re-add hard bounces to your list without re-validating them first. If you're migrating to a new ESP and want to use an old list, run it through validation before importing.
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