When Microsoft Outlook responds that there was an error connecting to an outgoing SMTP mail server, the issue can be related to several things including credentials, network or server issues.
Problems with your login credentials, including your username and password, may prevent Outlook from connecting to the SMTP server. Verify that your login credentials for the SMTP server have been entered correctly, including username, password, server name and port, if applicable.
Private networks, including many corporate networks and some Internet Service Providers, may block access to external mail servers. Consult your network administrator if you suspect that the Outlook error may be network-related.
If a server is not powered on, is off the network or is otherwise compromised, users cannot connect to it. An error message indicating that Outlook cannot connect to a server may indicate that the server is down.