What’s “disabled mailbox”?
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You send an email, and the bounce message comes back saying something like "mailbox disabled" or "account disabled." The address looks real, the domain is valid, but nobody's home. That's a disabled mailbox.
A disabled mailbox is one where the account still exists in the system but has been deliberately turned off. It's not the same as an address that never existed. The mail server knows about it. It just won't accept anything.
The most common reasons this happens are pretty predictable. An employee leaves a company and IT disables their corporate inbox. A user's account gets suspended for a policy violation or abuse. A hosting subscription lapses and the provider shuts the account down. Or a compromised address gets locked as a security measure.
The bounce codes you'll usually see are 550, 553, or the more specific 5.2.1, often with text like "mailbox disabled" or "account not accepting messages." Sometimes you'll see 5.1.1 used instead. The exact wording varies by mail server, but they all mean the same thing: delivery is permanently refused right now.
Should you treat it as a hard bounce? Yes. Suppress the address and move on. Could the mailbox come back? Technically yes (reactivated employee, renewed subscription), but you have no way to know when or if that happens. Waiting around and re-sending risks your sender reputation. If they really do come back, they can re-subscribe.
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