I am receiving this error message when trying to respond to emails.  My return email is not delivered.  I have not been able to find a clear answer searching the internet.  Any ideas??

Server error: '550 5.7.1 Relaying denied: You must check for new mail before sending mail.'

Go to File, Account Settings. On the Email accounts tab, double-click on the account then click More Settings. Look on the Outgoing Server tab.

Can you please tell me how this was answered?  I'm having the same problem.  Thank you!