Also, I have hit this problem several times with the new gateway and rebooted to find all messages are then accessed correctly. Have not seen any errors in other programs/apps.

with the error " Mail server mail.comcast.net respnded: Temporarily blacklisted IP Address - try again later." only comcast can help. It is their server sending the message that you are blacklisted from accessing their server. Why? I have no idea.

One error reads "The RETR command did not succeed. Error retrieving a message. Mail server mail.comcast.net responded: cellpadding="0" cellspacing="0"

The information is HTML layout code by the look of it. Possible from your mail, but quite possible from your web browser if the router/modem is defective. My wild guess is you got a lemon from the provider when it comes to modems.

Have you tried googling the model and manufacturer for similar issues? I recall a number of years ago it was Verizon or Comcast that sent out updated modems to their customers that blocked access to their own mail servers. So getting garbage configurations from your provider with their bundled modems has a precedent.

Thanks for response, Matt. When I spoke with tech support at Comcast and suggested they send me a new gateway/modem, the rep was not very receptive of that and (of course) suggested I should go through Thunderbird instead. After your analysis and due to this all being new problems since installing the new gateway, I may just request the provider to send me another of the older model gateway which seems to not have any of these problems.