How are you accessing the email? With only a browser and the webmail UI, or something else?
I use the web browser to click on xfinity.
OK, so if you open the Drafts folder are those messages in there? Can you close them out?
Try clearing your browser cache.
How do I do that?
What browser are you using?
Follow the directions here:
It seems you have max number of compose windows opened. (which is 10). Please close it and save it to drafts or discard it, if its not needed. Once you do, you should able to compose/send new messages.
Please let me know if you are still facing problems.