Ask why a domain that landed perfectly two months ago is suddenly hitting spam, and the answer is usually not a single dramatic event. It is decay. Sender reputation is not a permanent rating that mailbox providers assign once and remember. It is a continuously-updated assessment of how you have behaved recently, which means it is always either being reinforced or quietly eroding. Understand that, and a lot of confusing deliverability behaviour starts to make sense.
What reputation decay actually is
Picture your reputation as a moving window. Providers look at your recent sending and the engagement it produced, weight the most recent activity most heavily, and let older activity fade from the calculation. Keep producing positive signals (opens, replies, mark-as-important, spam rescues) and the window stays full of good evidence. Stop, and the good evidence ages out of the window with nothing to replace it. The score does not crash; it leaks. That slow leak is decay.
How fast it happens, Gmail vs Outlook
There is no universal decay rate, and you should distrust anyone who quotes one as a law. What is consistent is the shape:
- The pattern: inbox placement commonly starts slipping within roughly 10 to 14 days of inactivity and deepens the longer the gap runs.
- Outlook and Microsoft 365: lean heavily on sender reputation, so they tend to be less forgiving of a quiet spell. A gap that Gmail tolerates can cost more on Outlook.
- Gmail: weights ongoing engagement, so it responds quickly to a resumption of positive signals, but it also notices their absence.
Treat all of these as observed ranges that vary with the domain's age and prior strength, not as fixed numbers.
Why a one-time warmup fails
The logic is simple once decay is on the table. Warming builds reputation. Reputation decays. Therefore a warmup you do once buys you a healthy score that begins sliding the moment the engagement behind it stops. Teams that treat warming as a setup checkbox tend to see deliverability quietly erode a few weeks after they switch it off, and then misdiagnose it as a content or list problem. The actual cause is that they stopped feeding a score that needs constant feeding. This is also why a dormant inbox needs re-warming before you can safely use it again.
Continuous warming is the fix
If decay is caused by positive engagement stopping, the fix is to never let it stop. Always-on warming keeps a low, steady stream of genuine engagement flowing through your domain even between campaigns, so the rolling window is continuously refilled with fresh positive evidence and never empties enough to slide. MailStrike runs persona warming in the background at reduced volume alongside your real sending, and pairs it with reputation monitoring so a slip is caught and corrected before it reaches your campaigns. The point is not to warm harder. It is to never stop.
The short version
Sender reputation is a rolling assessment of recent behaviour, so it decays whenever positive engagement slows, typically becoming visible within about two weeks of inactivity and biting harder on Outlook than Gmail. A one-time warmup cannot hold, because the score it earns starts leaking the moment the engagement stops. The durable answer is continuous warming that keeps fresh positive signals flowing, so decay never gets a foothold. Warming is maintenance, not a one-off install.
Frequently asked questions about reputation decay
What is email reputation decay?
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Email reputation decay is the gradual decline of your sender reputation over time when positive engagement slows or stops. Mailbox providers do not store a permanent trust score; they continuously re-evaluate senders based on recent behaviour. So reputation is more like a rolling average than a bank balance: as older positive signals age out and are not replaced, your standing slips. It is not a punishment for doing something wrong. It is simply what happens when the evidence of trustworthiness gets stale.
Does sender reputation actually decay over time?
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Yes. Because providers weight recent activity most heavily, a domain that stops producing positive engagement drifts toward neutral and then toward suspicion. The effect is observable as inbox placement slipping after a sender goes quiet, commonly beginning within a couple of weeks of inactivity. Reputation also takes small hits from ordinary sending: every spam complaint, bounce, and ignored message is a minor negative signal that has to be outweighed by positive ones to keep your standing level.
How fast does email reputation decay?
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There is no universal rate, so treat any single number with suspicion. What is consistent is the pattern: inbox placement commonly starts slipping within roughly 10 to 14 days of inactivity and deepens the longer the silence lasts. The speed depends on the mailbox provider, how established the domain is, and how strong its reputation was to begin with. Microsoft and Outlook, which lean heavily on sender reputation, can be less forgiving of a gap than Gmail, which weighs ongoing engagement. Treat these as observed ranges, not guarantees.
Why isn't a one-time warmup enough?
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Because warming builds reputation, and reputation decays. A one-time warmup gets a domain to a healthy inbox placement rate, but the moment the engagement that earned that score drops off, the score starts sliding back. Teams that treat warming as a setup task they finish once are often surprised when deliverability quietly erodes weeks later. Warming is maintenance, not installation. The reputation you build has to be sustained, or it leaks away.
How do I stop my email reputation from decaying?
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Keep positive engagement flowing continuously. The most reliable way to do that is always-on warming at low volume, which maintains a steady trickle of opens, replies, and other positive signals even between campaigns, so older signals are constantly replaced rather than left to age out. Pair it with good list hygiene and low complaint rates on your real sending, and monitor your inbox placement so you catch a slip early. The goal is to never let the rolling score go quiet enough to fall.
Does reputation decay even if I keep sending?
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It can. Sending is not the same as positive engagement. If your real campaigns generate complaints, bounces, and low open rates, those negative signals erode reputation while you send, not just while you are idle. This is why volume alone does not protect a domain. What protects it is a favourable balance of positive engagement over negative, which warming reliably supplies and a poorly-targeted campaign actively undermines.