5.2.1 means the mailbox exists but is disabled, suspended or not accepting mail. It is permanent. Suppress it, and treat these addresses with more care than an ordinary hard bounce, because dormant mailboxes are frequently recycled into spam traps.
What it looks like in the log
550 5.2.1 The user's account has been disabled.
550 5.2.1 mailbox disabled, not accepting messages
Why it happens
The account was suspended or closed by the provider or the organisation.
The mailbox is dormant and has been switched off rather than deleted.
What to do
Suppress permanently.
Treat a rise in 5.2.1 as a list-age signal. These addresses are prime candidates for recycled spam traps.
Apply a sunset policy so unengaged addresses stop being mailed before they become traps.