I have the same issue and happen to notice you are reliant upon sympatico.ca as well. I wanted to blame Thunderbird but since Bell had major issues a few weeks ago, it might be them again.

pop account? what versoin are you using?

Also, What AV software and extensions are used?

It’s definitely TB I can read my emails fine on my iPad, on the Bt server and on my phone.

Just my desktop TB which is broken which is a disaster as it’s where I do 95% of my email work and has all my emails stored. (Which are fine - it’s just new one)

It seems like it’s a problem with the Bitdefender / Thunderbird- Presumably broken during yesterday’s update to Thunderbird.

I’ve turned off the “encrypted web scan” option on the online settings for Bitdefender and rebooted the PC and it all seems OK (found that on another thread). Not sure if it’s safe to do this so hopefully Thunderbird are going to fix it quickly. Don’t know if other anti-virus software is having a similar impact.

Had same problem and posted it. Through another person's help we also found that turning off 'encrypted web scan' in 'Online Threats Protection' in Bitdefender and reboot solved the problem.

Solution update posted at 8:48 Eastern time (12:48 UTC) Thursday by Alexlandru_BD, an admin at bitdefender, "The developers reverted the changes made in the last signature update and this should do the trick until a permanent fix is deployed. Just click on update in systray and reboot once completed. But keep in mind this change is being rolled out gradually, so it may not reach everyone at the same time.." More details at https://www.reddit.com/r/Thunderbird/comments/1mo693n/comment/n8ammvq/