If you are facing an error while sending mails through outlook as:

An unknown error has occurred. Account: ‘me@mydomain.com’ (’me@mydomain.com), Server: ‘mail.mydomain.com’, Protocol: SMTP, Server Response: ‘421 Too many concurrent SMTP connections; please try again later.’, Port: 25, Secure(SSL): No, Server Error: 421, Error Number: 0×800CCC67

Solution :
You will have to check for the settings in your outlook .
Click the check box next to “My outgoing server (SMTP) requires authentication”.
Also confirm that “Use same settings as my incoming mail server” is selected.