Email is not sending

My outgoing emails are not being received when being sent using the Xfinity.com email on my Safari browser. I do not use another email program...I access my emails directly by logging into my Xfinity account on Safari. When I compose an email and press send, it goes into the sent folder but the recipient isn't receiving the email. However if I send it to another comcast.net address, then it is received. I am receiving my incoming emails. What do I need to do for my outgoing emails to get to the intended recipient?