For example, the recipient might have been moved from an on-premises Exchange organization to Exchange Online, or vice-versa. You might have an invalid entry in your Auto-Complete list (also known as the nickname cache) for the recipient. Solution 2: Remove the recipient's email address from your Auto-Complete list To resend the message in Outlook, see Resend an email message. Check for correct spelling and send the message again if you find an error in the email address. It sounds too simple, but the wrong email address is the most common issue that causes 5.1.x errors. Solution 1: Confirm the recipient's email address If the steps in this section don't fix the problem for you, contact your email admin and refer them to the information in this article so they can try to resolve the issue for you. Here are some steps that you can try to fix the problem yourself. This article describes what you can do if you see error codes 550 5.1.1 through 5.1.20 in a non-delivery report (also known as an NDR, bounce message, delivery status notification, or DSN).
It's frustrating when you get an error after sending an email message. Mail flow rules are now available in the new Exchange Admin Center.
