Problems sending and receiving email messages can be frustrating. If you get a non-delivery report (NDR), also called a bounce message, for error code 550 5.1.8, this article can help you fix the problem and get your message sent.
Why did I get this bounce message?
You received this bounce message with error code 5.1.8 because your account has been blocked for sending too much spam. Typically, this problem occurs because your account has been compromised (hacked) by phishing or malware.
I got this bounce message. Help me fix it.
First, you need to reset your password and scan your devices for malware. However, the hacker might have configured other settings on your mailbox (for example, created Inbox rules to auto-forward email messages or added additional mailbox delegates). So, follow the additional steps in How to determine whether your Office 365 account has been compromised.
And, you need to tell your administrator that you think your account was compromised. Your administrator will need to unblock your account before you can send mail.
I'm an email admin. How can I fix this?
The sending account may be compromised. You will need to:
Determine if the account is compromised. If the account is compromised, follow the steps in How to fix a compromised (hacked) Microsoft Office 365 account.
Go to the Action Center to unblock the account. After you unblock the account, the user should be able to resume sending messages within a few hours.
To help prevent future account compromises, follow the recommendations in Security best practices for Office 365.