If you are using Exchange server then it is an authentication problem.you can try to create a receive connector for this server without Authentication.(remove all Authentication and select externally secured )
I experienced the same error on 6 out of 7 of our SQL Server Instances. I found a great article on Troubleshooting Database Mail but it had nothing to do with problem. The problem end up being that our Network Group had made some changes and forgot to open up Port 25 which is the port we use for sending mail. I hope this helps. Here is the link to the great troubleshooting steps: