550 5.4.1: Recipient address rejected: Access denied

Microsoft 365SMTP

The error usually looks like this:

550 5.4.1 Recipient address rejected: Access denied. AS(201806281) [...prod.protection.outlook.com]

What it means

The recipient’s Microsoft 365 organization doesn’t recognize the address you are sending to (Directory-Based Edge Blocking).

Common causes

  • The address is misspelled, or the mailbox has been deleted.
  • The recipient’s domain is set up in Microsoft 365 but the mailbox is hosted elsewhere and the domain is not configured as Internal Relay.

How to fix it

  1. Check the recipient’s address.
  2. If you are the recipient’s administrator, check the mailbox exists and the domain type is correct.

Tools to help diagnose it

SPF, DKIM, and DMARC checker

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

Email server settings finder

Find the IMAP, POP, and SMTP server names, ports and encryption settings for any email address

A free tool provided by Auth-Email

Auth-Email makes your Gmail, Outlook, or other OAuth 2.0 email account available to any IMAP, POP3, or SMTP email client — no app passwords or special configuration needed.

Read the docsTry Auth-Email →

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