Open your Outlook and go to Tools -> Accounts

At "Mail" column, the email accounts that configured in your Outlook will be listed.
Select the problematic email account and click on "Properties"

On the "Server" tab, verify your Incoming and Outgoing mail server name is correct.
Verify you have full email address on your Account Name with correct password.
Make sure the setting "My server requires authentication" has enabled.

On the "Advanced" tab, make sure the Outgoing mail (SMTP) port number is 26 (Tmnet/ Streamyx has block on port 25)

Finally, click Ok button to save your changes and restart your Outlook after that.