How to unblock 365 user accounts

Your message couldn't be delivered because
you weren't recognized as a valid sender.

Remote server returned 550 5.1.8 Access
denied, bad outbound sender AS(24004)

Common Errors:

Your message couldn't be delivered because you weren't recognized as a valid sender.

Remote server returned 550 5.1.8 Access denied, bad outbound sender AS(24004)

How to unblock a user in 365:

We have seen this issue crop up a few times of late. A 365 user account gets blocked from sending emails with the errors listed above. This is most likely because the user has been phished, their password is compromised and their 365 account is being used to send out spam emails to their own contacts.

First things first, if you are getting the errors above when trying to send emails you need to have your password reset ASAP. If you don't have admin 365 access then get your IT bods to reset your password for you.

Once your password is reset you need to go ahead and check your Outlook rules. This is important because if your account has been used for spamming it's likely the spammer will have created a rule so you wouldn't see the replies from the spam mails.

Check Outlook rules in Office 365:

  • Login online → open Outlook app
  • Click on the settings cogwheel (top right)
  • Click view all Outlook settings
  • Click rules
  • Check for any suspicious rules
  • Delete suspicious rules

How to unblock 365 senders:

  • Login online (with admin access)
  • Visit the URL
    https://protection.office.com
    /#/restrictedusers
  • View any blocked users
  • Click unblock on selected users
Your message couldn't be delivered because you weren't recognized as a valid sender.

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