550 5.5.0 (S2017062302) — Requested action not taken: mailbox unavailable (Outlook.com)
Retrying will not help. The message will not be delivered.
The reply
550 5.5.0 Requested action not taken: mailbox unavailable (S2017062302).Microsoft's way of saying there is no mailbox there. The wording sounds like a temporary condition and the S-code sounds like an incident number; neither is true. It is an unknown recipient, and it is permanent.
What it means
Despite the phrasing, this is the Outlook.com equivalent of a 5.1.1: the address does not exist, is closed, or has been deactivated for inactivity. Microsoft recycles neither. The S-code is a static reference to the reason, not a ticket, so there is nothing to quote to support and nothing to wait for.
Why it happens
- The address never existed — a typo, or a guess.
- The account was closed by its owner.
- Microsoft deactivated a consumer account left unused for a long period.
- A Microsoft 365 user was deleted by their tenant administrator.
- An alias that was removed from an account that still exists.
What to do
- Suppress the address permanently. There is no state here that changes on its own.
- Do not retry — the reply is final despite reading like a transient failure.
- Watch your rate of these. Microsoft weighs unknown-recipient attempts heavily when deciding whether to block the IP, so a bad list turns into an S3140 block.
- Add confirmed opt-in so addresses prove themselves before your first send to them.
Questions
Is S2017062302 a ticket number I can quote?
No. It is a fixed reference to this class of rejection and it is the same string for everybody. There is nothing to escalate.
The user swears the address is right.
Then check for a deactivated account — Microsoft switches off consumer mailboxes left unused for a long time, and mail to them fails exactly like this.