Outlook / Microsoft 365 delivery response
Understand Outlook / Microsoft 365 550 5.2.1: Mailbox disabled
The destination mailbox is disabled or unavailable Start with the complete response and the system that owns the failing step.
A representative response is:
550 5.2.1 Mailbox disabled.
Treat this as permanent until the complete provider response and retry behavior show otherwise.
What to preserve
Keep the complete SMTP response, timestamp, recipient, sender identity, queue or message ID, and every Received hop. Provider wording can narrow the interpretation, but a numeric code alone is not a complete diagnosis.
Ownership and next action
- Identify whether the sender, DNS operator, receiving provider, or recipient controls the failing step.
- Compare the exact envelope sender, visible From domain, recipient, and final receiving host.
- Follow the current provider documentation linked above before changing configuration.
- Make one narrow, reversible correction owned by the responsible administrator.
- Send a controlled test and retain the new response as evidence.
MailFixIt scope
MailFixIt can inspect bounded public DNS configuration on relevant guide pages, but it does not promise to clear provider reputation decisions, mailbox state, credentials, message-content blocks, or historical transport failures. Use related links only for the configuration they explicitly cover.
Frequently asked questions
What does Outlook / Microsoft 365 550 5.2.1: Mailbox disabled mean?
It is a receiver response associated with the destination mailbox is disabled or unavailable Use the complete wording and provider context before assigning a single cause.
Can MailFixIt prove this entire bounce from public DNS?
No. The current checker can evaluate selected public DNS configuration, but it cannot recreate recipient state, credentials, reputation, message content, or a historical provider SMTP decision.
Related email diagnostics
Primary sources: Microsoft 365 non-delivery reports.