SMTP error 550 5.2.1: mailbox unavailable

550 5.2.1 is a 5yz permanent failure with 5.2.1, which RFC 3463 defines as "mailbox

disabled, not accepting messages". This is a meaningfully different condition from 550 5.1.1

(mailbox does not exist): here, the receiving server recognises the address as a real, provisioned

mailbox, but it has been deliberately disabled, suspended, or otherwise blocked from receiving.

A real transcript

RCPT TO:<recipient@example.com>
550 5.2.1 Mailbox disabled for this recipient

Variants you will see across providers include "mailbox is disabled", "account is suspended", and

"mailbox unavailable" — all map to the same 5.2.1 condition.

How the big three phrase it

Note the Microsoft 365 variant: it reuses the 5.2.1 code for an object-count quota condition,

which is closer in spirit to a storage problem than a disabled account — always read the free text

rather than assuming 5.2.1 means exactly one thing everywhere.

Causes of SMTP error 550 5.2.1

• Mailbox is unavailable because an administrator has disabled it — common after an employee

leaves a company and IT deactivates the account without deleting it outright.

• Mailbox is over quota to the point the provider has suspended rather than just deferred it —

some providers escalate a long-running full mailbox from a 4xx deferral to this 5xx state.

• Disabled or suspended mailbox due to a policy violation, an unpaid hosting bill, or an

account under review.

Diagnose it yourself

1. If you administer the domain, check the mailbox's status directly in your admin console —

Google Workspace and Microsoft 365 both show a clear "suspended"/"disabled" flag distinct from

"active" in their admin UI, which resolves the ambiguity immediately.

1. If you do not administer the domain, check whether the bounce is isolated to one address or

affects an entire pattern of addresses at that company (e.g., everyone at a specific former

employer domain) — a mass disablement often follows a company-wide offboarding event or a

domain migration.

1. Check the timing against anything you know about the recipient — a 5.2.1 that started

appearing only after a long silence from that contact often correlates with a role change or

departure.

Actionable solutions for fixing SMTP error 550 5.2.1

Verify and fix mailbox settings

If this is your own domain and you administer the mailbox, check the account status directly in

your mail admin console — this is frequently a disabled-by-default state after account creation,

or a leftover suspension from an unrelated issue.

Check mailbox quota and increase capacity

Where the cause is quota-driven suspension rather than a simple full-mailbox deferral, raising the

storage limit or having the user clear space resolves it; this differs from 452 4.2.2 in that the

provider has stopped treating it as temporary.

Fix disabled mailboxes

For a recipient outside your control, there is no sender-side fix — contact the person through

another channel if the relationship matters, since your retries will keep failing identically.

As a sender, what to do

1. Confirm this is a permanent 5xx, not a 4xx deferral — check the full code before taking

action.

1. Remove the address from active sending; it will not become reachable through retries.
2. If this is a known, important contact, reach them another way rather than continuing to resend.

What not to do

• Do not confuse this with "address does not exist" (550 5.1.1) — the fix and the likely cause

are different, and conflating them in your bounce handling misclassifies the data and can hide a

genuine mailbox-provisioning problem behind a generic "bad address" label.

• Do not retry sending to this address; it is a permanent failure and will not resolve through

resending, no matter how many retry cycles your queue runs.

• Do not assume this reflects on your sending reputation — it is entirely a recipient-side account

state, and chasing an authentication or reputation fix in response wastes time on the wrong

problem.

MailPilot's delivery tracking records the specific enhanced status code behind every rejection, so

a 5.2.1 disabled-mailbox bounce is categorised separately from a 5.1.1 nonexistent-address

bounce and from a reputation-driven rejection — each needs a different response, and lumping them

together as "a bounce" hides that.