Comcast no longer exists as a mail entity. It has been AOL for some years now and with the usual creep that occurs when companies are merged AOL is getting more and more like Yahoo. In saying that I am pointing out that you can not have issues with Comcast mail, and when searching solution AOL is probably going to return more useful information than a search on comcast In this instance the sending address (whatever it is) does not match the user name on the authentication on the outgoing server (SMTP) entry in account settings. AOL require that the sending username match the sending address as the email address is used as the username in AOL. Go to outgoing server (SMTP) in your account settings and ensure you have one outgoing server for each and every address you wish to send from. lLease make sure the names of these account are clearly identifiable, as the next step is to ensure the correct SMTP server is used for each identify in your incoming accounts. Now go to your incoming accounts above the outgoing server(SMTP) entry and select the account name (this is the entry above the server settings) in the bottom right of the screen (you might have to scroll) you will see the outgoing server to be used named. Correct it as required. Click the manage identities button to ensure there are not additional identities in the account that require updating. Move to your next incoming account.