Hey Duncan, sorry to hear you're stuck with this Thunderbird mess after dealing with the antivirus stuff—let's work through this step by step to get your emails flowing again.

First, let's find where those email settings live in Thunderbird:

• Open Thunderbird. If you don't see the top menu bar, press the Alt key to bring it up. Then go to Tools > Account Settings (or you can click the three-line menu button in the top-right corner and pick "Settings", then select "Account Settings" from the left sidebar).
• Once you're in Account Settings, select your email account from the left list—this will show you all the key settings for that account.

Next, let's fix the TLS-related settings that are probably causing the error, since ClamAV might have messed with system or app security configurations:

• Under your account, click Server Settings (this is for your incoming mail server, POP3/IMAP).
• Double-check these critical fields:
  • Server Name: Make sure this matches what your email provider recommends (e.g., if you use Outlook, it's outlook.office365.com; for Gmail it's imap.gmail.com). If you don't remember, you can look up your provider's "incoming mail server settings" on their webmail help page.
  • Port: For SSL/TLS, IMAP uses 993 and POP3 uses 995. If you're using STARTTLS, IMAP is 143 and POP3 is 110. Stick with the port your provider specifies.
  • Connection Security: Don't leave this on "Automatic"—pick either SSL/TLS or STARTTLS (whichever your provider recommends). Avoid "None" entirely, since that's insecure and likely not allowed anymore.
  • Authentication Method: Most providers use "Normal Password" here; if you're using a major service like Google or Microsoft, it might be "OAuth2" (you'll need to re-authenticate if you switch to this).

Since you installed and uninstalled ClamTk/ClamAV, it might have left behind some certificate or proxy changes that are messing with Thunderbird's TLS connections. Let's check that too:

• Go back to Thunderbird's main Settings page (Menu > Settings), then select Privacy & Security from the left sidebar.
• Scroll down to the Certificates section, click View Certificates. Look for any certificates labeled with "ClamAV" or something related to the antivirus—if you find them, select them and click Delete.
• Also, check the Connection section under Settings: make sure "No proxy" is selected (unless you intentionally use a proxy, which it sounds like you don't).

If none of that works, let's try a fresh Thunderbird profile to rule out corrupted config files:

• Close Thunderbird completely.
• On Windows, press Win + R, type thunderbird -P and hit Enter. On Linux/macOS, open a terminal and run thunderbird -P.
• This opens the Profile Manager. Click Create Profile, follow the prompts to make a new profile, then launch Thunderbird with this new profile.
• Add your email account again using the correct server settings. If this works, your old profile was probably damaged by the antivirus install. You can then copy your old emails from the original profile to the new one (the Profile Manager shows where each profile is stored on your computer).

One last thing: double-check that your email provider hasn't recently updated their security requirements (like forcing TLS 1.2 or higher). You can confirm this by logging into your webmail account and checking their help section for incoming mail server settings.

备注：内容来源于stack exchange，提问作者Duncan Spears