I'm having the same problem (and a few others are on a couple other threads) sending from webmail using MacBook. It saves to draft folder. Then I can use the mobile Xfinity app and send from my draft folder.
Thank you to those who replied with your browser version, it was critical in assisting our trouble-shooting efforts.
We have identified an issue with older browsers before Safari 11 where sending a message saved the message to drafts instead of sending it.
If you have the most up to date browser the send issue will not affect you.
We realize that not everyone can, or wants to update to the latest browser and we have come up with a fix and are in the process testing so we can safely implement it overnight.
Thank you again for your assitance and your understanding.