If you are using Comcast's webmail, you can try using a feature called "Read receipts". The 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. To request a Read receipt in Appsuite, click Options in the Compose window, then "Request read receipt".
Many third party email programs and apps also support Read receipts, but . . .
Read receipts only work if both the sending and the receiving email software support them (many webmail systems don't), and typically only if the recipient grants permission to send them (many users never do). Given the limitations, consider including some version of "Please reply so I know you got this" with the message you send.