SMTP 554 5.7.1 · Outlook · Microsoft 365 · Corporate filters · SMTP

Message rejected due to content restrictions

A 554 5.7.1 means the receiving server's spam filter refused your message outright. What triggers it, how it differs from Gmail's 550 5.7.1, and the order to work through.

The bounce you received

554 5.7.1 Message rejected due to content restrictions
554 5.7.1 Service unavailable; Client host [203.0.113.10] blocked using Spamhaus

What the server is actually saying

The receiving server ran your message through its filter and refused to accept it. Unlike a 550 5.7.1 from Gmail, a 554 commonly comes from a corporate gateway - Proofpoint, Mimecast, Barracuda - whose thresholds are set by the recipient's IT team, not by a public provider. That means the bar can be arbitrary and the appeal path is the recipient's administrator.

Why it happens

• The sending IP or domain appears on a blocklist the gateway consults - Spamhaus being the most common.
• The recipient's gateway scores on different rules than the public providers, so content that passes Gmail can still fail here.
• Missing or failing authentication, which many corporate gateways weight harder than Gmail does.
• Attachment types or embedded links that the organisation blocks by policy regardless of sender.

How to fix it, in this order

1. Read the rest of the response. A 554 usually names the reason or the blocklist - that string is the actual lead, not the 554 itself.
2. Check the sending domain and IP against the major blocklists and request delisting where you are listed.
3. Verify SPF, DKIM and DMARC pass, since gateways weight them heavily.
4. If the message is legitimate and everything passes, the recipient has to ask their IT team to allowlist the sender - no sender-side change will clear a policy block.

Frequently asked questions

What is the difference between 550 5.7.1 and 554 5.7.1?

Both are permanent policy refusals. In practice a 550 5.7.1 most often comes from a large public provider like Gmail weighing reputation plus content, while a 554 5.7.1 more often comes from a corporate gateway applying its own rules - including blocklist lookups. The 554 response usually names the specific reason, so read the full text.

Can I retry a 554?

No. The 5 prefix means permanent - retrying sends the same message into the same refusal and adds to the pattern the filter is already judging you on. Fix the underlying cause first.

Related errors

• 550 5.7.1 — Our system has detected that this message is likely unsolicited mail
• More bounce errors