What’s “disabled mailbox”?

A disabled mailbox is intentionally deactivated:

What it means:

Account exists but is not accepting mail. Administrator or provider disabled it. Different from non-existent user.

Common reasons:

Employee departure: Corporate account disabled. Account suspension: Policy violation or abuse. Non-payment: Subscription or service lapse. Security: Account compromised and locked.

Typical codes:

550 or 553 with "mailbox disabled" or "account disabled." 5.2.1 (mailbox disabled, not accepting messages). 5.1.1 may also be used.

Handling:

Treat as hard bounce. Suppress the address. Account reactivation is possible but unpredictable.

Disabled mailbox is a closed shop. The address exists but is not open for business.