The address you sent your message to couldn’t be found at the destination. It might be misspelled or it may not exist. Try to fix the problem by doing one or more of the following:
Retype the entire email address manually and resend it – if your mail program automatically suggests an address to use don’t accept it.
Contact the recipient by some other means (by phone for example) to confirm you’re using the right address. Also ask them to check that that any mail forwarding they've set up is working correctly.
Clear the recipient nickname cache in your mail program by following the steps in this article: Status code 5.4.14 in Outlook.com and Office 365.