For the last fews weeks, emails are sporadically not received by recipients. The recipients are not using the same email service. The emails are in the Sent email folder.
This has happened previously for a few weeks and then the problem stopped, apparently on its own.
I can confirm that the emails are leaving the Comcast email server. If the recipient is not receiving your emails, have them check their spam folder and filtration settings as something is likely filtering out your emails.