I have been having the same problems...getting emails but not being able to send via Windows Live Mail. The link below really helped me. I didn't have the "use the same as the incoming email" checked. I checked the box, had to restart the program and was finally able to send an email. I deleted the messages sitting in my outbox and copied them into new mail just to make things easier and to check the system. This has been very frustrating and the worst part was being on the line with Cox customer service and they sent me over to a paid help line citing Windows as the problem. Ugh
Good Luck!
For Windows Live Mail settings, refer to https://www.cox.com/residential/support/changing-your-email-server-settings-for-windows-live-mail.html.