ComCast

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ComCast is known to block port 25 (SMTP) you need to change your account  setting in oulook for your OUTBOUND SMPT Port to port 66, it has a default  for 25 as installed.      Advanced has had port 66 open as a reserve for all clients for over 10  years,    If you have Outlook and do not know how to setup your port, please use the  'help' function of your version of outlook.