ARe you using ESET Anti-Virus ? If no then what Anti-virus are you using?

If yes: ESET AntiVirus has been known to cause issues with downloading emails.

• https://support.eset.com/en/kb7728-unable-to-access-or-receive-emails-in-thunderbird-with-eset-product-installed

Temporarily Disable ESET SSL/TLS Filtering:

• Open ESET's main program window.
• Press the F5 key to access Advanced setup.
• Click "Protections" and then expand "SSL/TLS".
• Disable SSL/TLS filtering temporarily.
• Re-enable it after Thunderbird is working properly and restart your computer.

Exclude Thunderbird from SSL Filtering:

• Open ESET's main program window.
• Go to "Advanced setup".
• Expand "Web and Mail" and then "Web and E-mail Protections".
• Click "Exceptions".
• Add an exception for Thunderbird's executable file.
• Click "OK".

What server are you using? gmail, BT, Yahoo, comcast etc

What server are you trying to connect to ?

Are you using ESET? If no what Anti-virus are you using?

Thanks for replying. We use SOPHOS antivirus which has never caused any issues, so we doubt that is the problem. Email is provided by Mailfence. They had blacklisted our account due to suspicious activity but it is still not cleared. Not sure if this is a Thunderbird or Mailfence issue.

re :They had blacklisted our account due to suspicious activity but it is still not cleared. Not sure if this is a Thunderbird or Mailfence issue.

Well that explains everything. If account is blacklisted that might be email address or your IP address. This has nothing to do with Thunderbird. If Mailfence have said they blacklisted your account then you need to know how they are going to remove the listing.

Suspicious activity is a somewhat broad definition. It might be spam content in emails, but first check to see if you have been reecntly using a VPN because that will cause servers to block accounts. If you have used a VPN then server will not like it because it hides your IP address and they might think you are a hacker, so check your computer for a VPN which has been enabled.

SOPHOS antivirus does provide a VPN. Access the Antivirus and switch off the VPN or you need to use the slpit tunnelling and set it up so Thunderbird uses it. You may need to contact SOPHOS to get more info on that.

I located this info:

• https://docs.sophos.com/nsg/sophos-firewall/20.0/Help/en-us/webhelp/onlinehelp/VPNAndUserPortalHelp/VPN/index.html
• https://docs.sophos.com/nsg/sophos-firewall/19.5/Help/en-us/webhelp/onlinehelp/AdministratorHelp/RemoteAccessVPN/HowToArticles/RAVPNSSLSplitTunnel/index.html

You could also check at this link to text your email address or domain or IP address. https://check.spamhaus.org/