I'm going to assume that bypassing the Comcast and moving to your own domain served from a different commercial hosting provider is not an option. (That approach usually has better support.)
There are various reasons servers will drop mail, or will not forward mail, or the mail forward from the servers will get dropped. That logging and that information is not generally visible to a client of the mail servers, unfortunately.
There's either something wrong with the Comcast servers, or your setttings differ between your OS X system and your iOS devices.
Does mail sent to yourself on the Comcast servers with your Comcast email address get received, or does everything sent from OS X to that mail server disappear?
With OS X, make certain that the selected SMTP server is the appropriate one for Comcast, as an account entry on OS X can have various mail servers, ports and SSL/TLS or not. (You're probably aware that there are two separate passwords and two separate connections — the receive path via POP or IMAP and the send/SMTP path — with each account, but it's worth verifying the settings. It's unfortunately easy to get the SMTP servers settings tangled as they're fairly well hidden in most account setups.)
One common brute-force option is to delete and re-add the entry. Remove the POP/IMAP account and the SMTP server account, then clear out any of remaining passwords that might lurk in the Keychain for the POP/IMAP receive server and SMTP server send path, then re-add the account in Mail.app.
