Comcast Email Issues - Can't Send Emails

I have Comcast as my service provider. About three times now, I have had an issue with Comcast not allowing me to send emails. I can receive them, but not send them. I have had to delete the Comcast account from mail and make a new one. This has not only been on our Mac's but our iPhones as well. I have gone through several technicians that say it is on our end, but I don't believe it is.

Has anyone else had this issue, and did you find a solutions.

Thanks!