Why aren't people receiving my emails?

I host email on a 3rd party server for hundreds of clients using the Smartermail Application. A handful of clients, using comcast as their ISP, cannot send email directly from our webmail service. They will login to the webmail, compose and send the mail and the email is never delivered. No bounceback is received and no error message is shown. The only way they know that it is not delived is because their clients are complaining. I had them try to send me a test email to hotmail and yahoo but I never received the messages.

I am on comcast at my office and I can login to their webmail and send the message to yahoo and to hotmail and any other account and it is delivered just fine. It appears it has something to do with their comcast connection not allowing email to leave from their office.

Comcast support has not been able to help at all with this. Any clues would be greatly appreciated!

Thanks,
Patrice