5.1.8 Microsoft bounce: outbound sender blocked

What 5.1.8 means

The sending account was blocked by Exchange Online for sending too much spam, usually because it was compromised.

Message text

Access denied, bad outbound sender

Common causes of 5.1.8

• Account was compromised by phishing or malware and used to send spam

How to fix 5.1.8

1. Reset the account password and review its sign-ins and forwarding rules.
2. Contact Microsoft support to restore sending once the account is clean.

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Have the full bounce message? Paste it into the bounce code lookup and it will match the code and the provider for you. Open the bounce code lookup.

How to read 5.1.8

The second part, 5.1.8, is an enhanced status code (RFC 3463 §2) written class.subject.detail. Class 5 is a Permanent failure (5xx): the command was rejected and the sender should not repeat it unchanged (RFC 5321 §4.2.1; RFC 3463 §2).. Subject 1 is addressing: a problem with the recipient or sender address. The detail number refines the subject, and providers may define details of their own, which is why one code can read differently at Gmail, Microsoft and Yahoo.

Do not keep resending the same message: a permanent failure repeats until the cause is fixed. Correct the cause, then send a new message.

Official source: learn.microsoft.com. Last verified 2026-10-20.

Related guides

• How to set up MX records (priorities, null MX, migration)
• MX host does not resolve: causes and how to fix it
• Null MX (RFC 7505): publish it for domains with no mail

Related codes

• 5.1.1 SMTP: bad destination mailbox
• 5.1.2 SMTP: bad destination system
• 5.1.3 SMTP: bad address syntax
• 5.1.10 SMTP: recipient has null MX
• 550 5.1.1 Microsoft: bad mailbox address
• 550 5.1.10 Microsoft: recipient not found

FAQ

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