550 5.7.606–649: Banned sending IP

Microsoft 365 / Outlook.comSMTP

The error usually looks like this:

550 5.7.606 Access denied, banned sending IP [x.x.x.x]. To request removal from this list please visit https://sender.office.com/

What it means

Microsoft has added the sending server’s IP address to its own block list.

Common causes

  • The IP address has sent spam, or shares a reputation with other senders that have.

How to fix it

  1. Request delisting at sender.office.com.
  2. Check whether the IP is also listed on public blocklists.

Tools to help diagnose it

Email blacklist checker

Check whether a mail server IP address or domain is listed on common email blocklists

SPF, DKIM, and DMARC checker

Check your domain’s email authentication DNS records, count SPF lookups and diagnose deliverability issues

A free tool provided by Auth-Email

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