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For detailed instructions, go to your antivirus/firewall official support page. The steps to follow are different depending on the antivirus/firewall you’re using.

If you can send and receive emails with the antivirus and firewall off, whitelist Thunderbird. To test if your antivirus or firewall is blocking outgoing emails, simply disable the tools.
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Or they could be blocking the emails coming to your inbox. Your antivirus or firewall might be preventing Thunderbird from delivering your messages. Whitelist Thunderbird or Disable Your Antivirus/Firewall
