Thunderbird won't download new messages

> opening thunderbird triggers quarantine of newmsg file as heuristic.

Please provide more details?

Turn off real time scanning or temporary disable your antivirus program.

Been running McAfee for years and suddenly out of the blue it starts flagging 'newmsg', which I understand is a temporary file created by Thunderbird during the download of incoming emails, which can be scanned by antivirus software if configured correctly.

Thing is, I never configured Thunderbird as such, nor can I find the setting to do so. Hope this helps.