I have on occasion run into the error in the past and had found usually that it was a bad message in the Inbox on the ATT/Yahoo mail server. But this time there is no apparent bad message. I moved all of the messages in the Inbox to another folder and attempted to get the new messages with the same error message. I have created a new profile and using that new profile with only the one email address in Thunderbird (TB) I still received the same message. I performed the modification recommended for Norton Antivirus users about excluding the entire profile folder from being checked and still got the same error message. So I am hopeful that someone else can provide some insight as to how to fix the issue. FYI I set the forwarding feature in the settings for the ATT/Yahoo email to forward to another email address that I also have set-up in TB and it received the messages without any issue. So only the regular email account is affected. I have a sub-account from that main email account on ATT/Yahoo and TB received messages sent to that email address without an issue. I am using the most recent version of TB v45.6.0. Thanks.