I can also send email from the command line using telnet to the ISP's mail server on port 25. This always works. Occasionally an email sent through Outlook will go.

Fix Outlook Error Receiving reported error (0x800CCC1A), Your server does not support the connection encryption type you have specified, Try changing the encryption ...

Definitely check your 'sent' folder on the Yahoo web client (assuming it is Yahoo you send through) to make sure you are not sending mail you don't intend.

If you get error 0x8007054e when adding account to Mail or Calendar app, restart you computer, reset Mail or Calendar app. Read on.