If you have "Zimbra" webmail the address in the browser will be "web.mail.comcast.net/zimbra/..." and you won't have webmail Read receipts. If you have the newer Open-Xchange "Appsuite" webmail the address in the browser will be "connect.xfinity.com/appsuite/..." and Read receipts will be available. To request one, in the "Appsuite" Compose window, click Options, then "Request read receipt".
Many third party email programs and apps also support Read receipts and you can use them whether your webmail is "Zimbra" or "Appsuite", but . . .
Read receipts only work if both the sending and the receiving email software support them (most webmail systems don't), and typically only if the recipient grants permission to send them (many users, like me, never do). Given the limitations, consider including some version of "Please reply so I know you got this" with the message you send.