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?
If the sender who got this bounce message can't fix the issue, follow these steps:
Take the actions described in How to fix a compromised (hacked) Microsoft Office 365 account.
In the Office 365 Security & Compliance Center, view the security alerts and unblock the compromised account. The unblocked account should be able to resume sending messages within one hour.
To help prevent future account compromises, follow the recommendations in Security best practices for Office 365.