550 5.4.1 — Recipient address rejected: Access denied (Microsoft 365)
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The reply
550 5.4.1 Recipient address rejected: Access denied.A Microsoft 365 tenant refused the message. This is not the consumer block list and not your reputation — it is a rule on the receiving organisation's own mail settings, and only their administrator can change it.
What it means
Common enough to be worth knowing by shape: the address usually exists, but the tenant's configuration will not accept mail at it from outside. That may be a directory-based edge block rejecting an address not in the directory, an internal-only mailbox, a blocked-sender rule that names you, or a distribution list restricted to internal senders. No sender-side change touches any of them.
Why it happens
- The address does not exist in the tenant's directory and directory-based edge blocking is on.
- A mailbox or group configured to accept internal mail only.
- The tenant has your domain or address on a blocked-senders list.
- A transport rule at the organisation rejecting mail matching something about yours.
- The mailbox was deleted but the address still resolves through the tenant.
What to do
- Contact the recipient through another channel and ask their IT administrator to check — you cannot resolve this from outside.
- Have them confirm the address is spelled correctly and accepts external mail at all.
- Ask them to check blocked-senders and transport rules for your domain.
- Verify SPF, DKIM and DMARC anyway, so the message is not caught by a rule about authentication.
- Suppress the address if the organisation will not or cannot accept the mail.
Questions
Is this my reputation?
Almost never. It is a per-tenant configuration decision. The same message from the same IP will reach other Microsoft 365 organisations normally.
Can I appeal to Microsoft?
No. Microsoft hosts the mailbox but the organisation sets this rule, so the sender support form has no bearing on it. The only route is their administrator.