You can try using an email feature called "Read receipts". Comcast's older "Zimbra" webmail ("web.mail.comcast.net/zimbra/...") does not support Read receipts, but the newer Open-Xchange "Appsuite" webmail ("connect.xfinity.com/appsuite/...") does. In the "Appsuite" Compose window, click Options, then "Request read receipt".
Note that Read receipts only work if both the sending and the receiving email software supports 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.