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Microsoft 365: Fixing error 550 5.1.8 when sending emails

This article applies if you have one or more of the following issues:

  • You are suddenly unable to send emails from one of your Microsoft 365 mailboxes.

  • When you try to send an email, you receive a non-delivery report (NDR) with an error message similar to the following:

    550 5.1.8 Access denied, bad outbound sender

This article explains why Microsoft 365 blocks a mailbox from sending emails with error 550 5.1.8, and how to unblock the mailbox in the Microsoft Defender portal once you have secured the account.

Why Microsoft 365 blocks a mailbox

Microsoft 365 automatically monitors all mailboxes for suspicious activity to prevent misuse and protect email deliverability for all customers. Typical triggers for a block include:

  • an unusually high number of emails sent within a short period of time
  • sending emails that show typical characteristics of spam
  • signs that unauthorised third parties have gained access to the Microsoft 365 mailbox (e.g. following a phishing attack or because of a compromised password)

If Microsoft detects any of these patterns, it automatically adds the mailbox to its list of restricted senders, which administrators can view on the Restricted entities page. The mailbox can still receive emails, but it cannot send any until it is manually unblocked.

Important steps to take before unblocking your Microsoft 365 mailbox

Before you remove the block, check whether the emails that triggered it were actually sent by you or by an authorised user.

If you are certain that you did not send these emails yourself, unauthorised third parties have probably gained access to the account. In this case, complete the following steps before you unblock the mailbox:

  • Change the password for the affected user account immediately.
  • Enable multi-factor authentication (MFA) for this account if you have not already done so. With MFA, a stolen password alone is no longer enough to access the mailbox.
  • Check whether the account has any unknown forwarding rules, changed contact details, or unknown registered devices, and remove them if necessary.

Only unblock the Microsoft 365 mailbox once you have secured the account following the steps above.

Unblocking a Microsoft 365 mailbox in the Microsoft Defender portal

You do not unblock the mailbox in the IONOS systems, but directly in the Microsoft Defender portal. To do this, complete the following steps:

Requirements

To complete these steps, you need a user account with the Global Administrator or Security Administrator role. Without one of these roles, you cannot unblock mailboxes on the Restricted entities page.

  • Open the following page in your browser: https://security.microsoft.com/restrictedentities
  • Log in with your administrator account.
  • On the Restricted entities page, you will see all the mailboxes in your tenant that are currently blocked from sending emails.
  • Select the checkbox next to the mailbox you want to unblock.
  • Click Unblock.
  • In the Unblock user panel, read the details on the Overview page and check the recommendations to make sure the account is no longer compromised. Then click Next.
  • On the next page, you can enable MFA and reset the user's password if you have not already done so. Then click Submit.
  • In the warning dialog, click Yes to confirm.

In most cases, the restriction is removed within one hour. In rare cases, it can take up to 24 hours before you can send emails from the mailbox again.

Preventing mailboxes from being blocked again

To prevent your mailboxes from being blocked again, we recommend taking the following measures on an ongoing basis:

  • Secure all Microsoft 365 mailboxes with multi-factor authentication (MFA).
  • Use a unique, sufficiently complex password for each account, and change it immediately if you suspect any misuse.
  • To avoid phishing attacks, do not open links or attachments in emails from unknown or unexpected senders.
  • Regularly check the sign-in activity and security settings of your Microsoft 365 mailboxes.

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